All sizes are pretty nominal: '6x7cm' runs from 55 to 57mm on the short dimension, though most are 56, and 68 to 72mm on the long dimension, and I've measured full-frame 35mm from 22.5 to 25 on the short dimension and 35 to 38 on the long dimension.

In the days of uncoated enlarger lenses, undersize carriers may have been an important safety measure in reducing enlarger flare, and they just never got changed.

It also seems that a lot of people don't WANT to print all-in.