That's hard-core. OTOH, tires tend to stretch over time too so what was once a 25mm tire might become 27mm after a couple years of being inflated at 100 psi.

Of course, tire pressure, rim width, and rider weight make small differences as well.

Some of my computers (I'm looking at you, Cateye) can only be calibrated in cm, so they have up to 0.5% error built in.