@NathanKnudson already mentioned the well in the middle of tubeless rims. This merely illustrates why it's necessary to use that well. The picture below is from Slowtwitch.com.
For readers not used to tubeless rims, the well or center channel has a narrower diameter than the rim's bead shelves, where the tire beads sit when inflated.
When I mount the first side of a new tire, I get that side's bead in the well. Then I work the other bead across the rim's sidewall. If I get stuck, I go around the rest of the tire and make sure both beads are pushed into the well, then I return to the bit of bead that I couldn't move earlier.
I don't follow rim standards closely. However, in 2020, the ERTRO clarified the bead seat dimension and tolerance for tubeless rims at 621.95 +/- 0.5mm. I'm not sure what the standard and tolerance was for non-tubeless rims. I recently listened to a Cyclingtips podcast (which I believe but am not 100% sure is this one) where, if I recall correctly a wheel industry engineer did say that before the above tubeless standard was adopted, some rim manufacturers may have made their rims with slightly higher than normal BSDs to give tight fits. For example, I think I recall that many Stans rims were reputed to be tight, and I owned a pair of Pacenti SL23s which had a similar reputation. Similar issues do exist on the tire side as well. I have heard, as he alludes to, that Conti tires are relatively tight on average. Last, I don't know what the official BSD dimension and tolerance was for non-tubeless rims. Specifically, I don't know if the mean dimension was smaller than 621.95mm, or if the tolerance around the mean dimension was larger, or both.
And the issue of mean dimensions is separate from the issue of manufacturing tolerances, which are technically the amount by which something is allowed to vary from the mean. In the lay usage of the word tolerance, people tend mean the amount of variation that a specific manufacturer has, i.e. some manufacturers have higher manufacturing variation aka worse tolerances than others, and on average reducing variation increases cost.