Why do cartridge manufacturers give a range for VTF?

My Sussurros'  manual says 1.8 - 2.2 grams. I know that Lyra, particularly the Delos has it rated at a very specific number. Just wondering, It would seem that the stated range would be very narrow.
So 2.0 grams +/- 10% doesn’t sound too bad for a reasonable range. What is the accuracy of your VTF gauge and turntable setting?
Most people think of VTF in terms of stylus force. Because that's what they see, that's what wears, that's where all the action is, that must be what it's all about, right?

Wrong. Really VTF is all about what is going on at the other end of the cantilever.

Way up at the end inside the cartridge are the coils of your moving coil cartridge. Those coils are held within a powerful magnetic field. Its the coils moving within the field that generates the signal we amplify and hear. 

Now this magnetic field, its not very big at all. Especially not the part of the field that has been engineered to be uniformly strong. If the coil moves too far off center one way or another its going to affect the linearity of the signal.

Got it?

So VTF is spec'd to place the coils within that field. How much it can move around and still be sonically up to spec depends on the particular details of each cartridge design. 

Now other things being equal we could say Lyra has engineered a microscopically small uniform field area with near zero VTF tolerance. Or we could say they really do have about the same as everyone else but for whatever reason prefer to appear precise. Or others cut their customers more slack. Whatever. Point is you can't judge a darn thing from this number.

*For MM instead of MC, same deal, simply swap coil for magnet.
@celander ,

I'm not having any issue, just a question for my own benefit.


Your explanation makes good sense. Thank you sir.
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Well, normally there is a small range from any manufacturer, but there is also an exact recommended tracking force in any manual i have seen (no matter MM, MI, MC ....). There is also recommendation to burn in a new cartridge with higher tracking force and then lower it down to recommended tracking force. A used cartridge with weak suspension can be used with lower tracking force. So there is a case for almost any situation.  I always use recommended tracking force for 99% of my carts. 
I've noticed that there seems to be a post removed almost every time I post. Do I have a stalker? A'gon is looking into it.
Agree with Chak that most carts have a range but then there will usually also be a recommended number, invariably pretty much in the middle of the range.

I just shoot for that middle ground number every time or slightly higher.
I think too the tone arm the cartridge is bolted onto is a consideration. Perhaps one with less friction in it’s bearings can enable the cartridge to track at lower VTF than one with slightly higher friction. Cartridge manufactures may specify a tracking force range in acknowledgement of that reality.
I think it should be a specific number....there is only one spot that aligns the coils et al as it was designed. 
But after you set static VTF, in actual use the cantilever is moving constantly. How much is to some degree affected by tonearm inertia. So there is no one exact correct value for VTF. Also no two samples of any cartridge are exactly alike in terms of compliance. Another reason to provide a range of values.
Lyra Delos also has a VTF range; it's 1.7g - 1.8g with 1.75g being recommended. 
All that means is that they want you to start at 1.75g and move on from there. This is no different than any other manufacturer 
I agree with stringreen's argument while the reason is my Allaerts
MC 2: 1,8 g ( maximal tolerance 0,05g). However chakster's
''damping argument'' should also be of some importance because
the damper is behind the coils and, depending from its ''hardness''
should have some influence on the VTF. In my opinion Allaerts
demonstrate that getting optimal position of the coils is not an 
easy proposition. J. Carr wrote about this problem as well his
solution to the problems by his new cartridges. His intention is
also to get those ''tolerances'' as small as possible. 

I've noticed that there seems to be a post removed almost every time I post. Do I have a stalker? A'gon is looking into it.

To ease your paranoia, at least in this case, it was me who deleted my own post when I realized I was reflecting on VTA and not VTF, therefore I was writing something totally irrelevant in a moment of brain fog.  I'm not saying you don't have a stalker but in this case it was just me ;-)
t_e_p, how did you delete your post?

Recently I posted a question, then the next day found my answer.  So I attempted to delete it but could not find a way.  No one had responded to it yet.  All I could do was erase the content and mark it "removed".
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Threads themselves can be deleted and removed by the OP for sure, unless Audiogon changed that in last few days.
Done it myself on more than one occasion.
In most cartridges the coils sit between two magnetic poles. The magnetic energy between  the poles is even so position within the gap is insignificant. The only exception I can think of is one Van Den Hul model that only has one pole. So the recommended VTF is a trade off between tracking ability and stylus/record wear. Again the test record is the ultimate way to determine the right VTF using the manufacturer's range as a starting point. I will use the lightest VTF that makes it through the tracking band. Sometimes this lies outside the recommended range. So be it. There are too many variables for the manufacturer to be able to predict.  What ever produces the best results is the right VTF. Without a test record you are just guessing. 

I deleted my post within 2 minutes of posting it.  I think my options were "edit" or "delete" and I chose delete.