The instrument used can read ±.0001
Variation on one recent set was ±.0002. Again, definitely not the end of the world, but variation is always there. I’m not trying to scare anyone into thinking their stock engine is going to fall apart because of a .0001 difference in piston to wall clearance. The .0002 variation was acceptable.
On the other hand, some manufacturers i.e. Subaru have A, B, and C pistons for one model block casting so they can match a piston to each bore from the factory. We certainly don’t want to take less care than the factory does since we’re building engines that will be subjected to higher RPMs, greater cylinder pressure and heat, etc. than a stock car was meant to see.
On some V8 setups a performance build still may have looser tolerances than some factory 4 cylinder tolerances and I’m guessing that’s why Jesse was surprised.