Why do cartridge manufacturers give a range for VTF?

@slaw 

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 ;-)

@pryso 

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