Strange how many used valves measure NOS?

Finally fired up my valve tester.
All I can say is be very afraid.
Out of boxes & boxes of used valves bought on here
and ePain, most of them are crap.
I don't mind the odd chance buy being duff but some bought from here cost a fair amount and I'll be taking "measures as NOS" with the pinch of salt it deserves on ebay from now on.
There's either a lot of uncalibrated valve testers out there
or a lot of lying b------s.
I think it may be the case that some of these have been tested many moons ago by the original owners, marked up then used for a few years then the resellers think since it's marked 90/90,
it's still nos.
Bizarre, I have definite NOS 6SN7's that struggle to make 90/90.
Still it's fun to discover the occasional diamond amongst the rough.
Yes I will be getting the tester's calibration checked but having done dozens of valves and half a dozen different types, I can't see that it can be far out.
We live & learn.