A manufacturers specification comes from testing that is more complex than simply navigating an oval of a certain size. It also includes more complex situations with S curves, switch machine clearances and so an and so forth. As such, something rated for O36 may successfully navigate a simple O31 oval, but collide catastrophicly with a switch machine or not have the truck rotational span to pass through an S curve or so on and so forth. Thus it gets rated O36 as it passes the oval, switch machine, S curve and other tests at that curvature.
To put it another way, imagine you purchased a locomotive rated O31, but it can't navigate your layout's S curve or go through it's switches? You'd be unhappy it was rated O31 and not O36.
