[Imp]

We know the correct information for older equipment, it either becomes declassified or some one does tests on captured or procured equipment.
Stats are then modified accordingly if needed, though normally they were not far off in the first place.
Sure there is the odd error creeps in more due I am guessing to the volume of data & data entry problems than anything else.
Based on this one can say even the modern stuff is pretty close, minor tweaks when hard data is available.
They are based on experts evaluations not some guess.

Thousands of hours have been spent by people making sure as best as possible things are accurate.
Its fine if you want to play with a fictitious OOB but state clearly that is what it is & it bears no resemblance to real equipment.

As an example for years people from time to time would moan Russian tanks are to tough, T-72 T-80 etc.
Tests conducted on them have since proved the game was virtually spot on.