It would be interesting to measure the actual diameters of rims that are tough mounts vs easy mounts.
That kind of accuracy at home would be tough. Few of us have calipers that open wide enough and with jaws deep enough to span a rim and clear the hub. With a tape measure, you are bending around the hub and probably spokes. But a non-digital approach would be to nail two (say 4") pieces of 2x4 to a wall or tabletop 13 1/3" apart. Drive two nail vertically so a rim touches them. Now a tight rim will be a push to get through and a small one will show a paper width of space. (You do need to check that your rim is actually round first. A tiny bit of hop will show bigger than what we need to measure to establish tire mounting ease. And that hop doesn't change the circumference; the only dimension that really matters for tires.)
I'll leave this to someone else. My Vittoria tires mount easily on my non-tubeless ready Open Pros and when those rims expire, I'm going back to tubulars.