The most natural way for shoe mismatches is from the showroom. Customer grabs 5 pairs, tries some on, they get mixed up when returning to stock. Same shoe and size (hopefully, lol), but different production number and different factory/origin.
GP didn't give enough context ("MY shoes are weird so I extrapolated" vs "I know this is true; here's proof."), but the casual one-off scenario is absolutely a showroom level of "random happens".
Babysitting all those individual shoes and styles and sizes seems ... expensive, logistically.
In fact, if this were a thing, I'd imagine they'd start offloading them as though it were a cool feature. "Want to mix-n-match shoes? buy just a left!"
There was a Seinfeld joke to this effect that men just take whatever clothing, even if it barely fits. Women are likely to be choosier and insist on a good match. Some might intentionally mix and match shoe sizes if one foot is bigger than the other (leaving the store with a mismatched pair)