Ask Kenda, they make consistently and accurately sized tyres, which is completely shocking in this industry... Worst inaccuracy I've had from them was a 2.1 that measured up 2.0. That's across the width, including knobbles, which is the only sensible and practical way to do it.
It's really obvious a lot of tyre companies are just lying, the tyre "size" gets chosen by the marketing men presumably... "OK, here's the new Highroller, * it measures 2.15 across, so let's call it a 2.15" says the designer. "No!" says the marketeer, "Nobody'll buy a 2.15 trailbike tyre that weighs this much, we'll say it's a 2.35"
Conti made that heroic attempt to pretend they just use a different standard, what was it, "This is the measurement of the treaded part of the tyre, from side to side, when the tyre is laid out flat". But then people impolitely pointed out that their tyres didn't meet that standard either.
And then you get, "The new Rubber Queen 2.4 is huge". Is it really? I measured mine, it was 2.4 inches across. But people expect Contis to be completely crooked sizewise so when they come up accurate, it's a shock- to the extent that some people couldn't even get them in their frames, because they either believed their 2.4 MKs were 2.4, or they assumed all 2.4s would be undersized.
It's all a load of balls tbh. But it does seem to be getting a wee bit better doesn't it? The new Contis and Maxxises are mostly very close frinstance. Course that's just even more confusing since the 2.25 Ardent is bigger than a 2.35 Highroller.