Why?

[VolunteerK9]

Maybe to tempt beekeepers to ask "Why"? :-D

Maybe the best answer yet. You know a beek always has that question in the back of his mind when it comes to the ladies. Why do they do what they do.

Yup, and probably the why and wherefore we as beekeepers are constantly striving to reinvent the wheel

[Michael Bush]

The original standard sizes were:
9 5/8
6 5/8
5 3/4
4 3/4

Frames

9 1/4
6 1/4
5 1/2
4 1/2

This leaves 3/8" on the bigger boxes (max beespace) and 1/4" on the smaller boxes (min beespace).  But people keep cheating.  Most deep boxes are now 9 1/2 or even 9 1/4".  Most deep frames are now 9 1/8" or even 9".  I think it was a compromise to get close to beespace on the frames while cheating on the boxes to get them to come out of a one by ten.

[David McLeod]

Mr Bush, that makes perfect sense. Since the deep has shrunk with the shrinking standard dimensions I can see why manufacturers have shrunk the frames. I kind of wondered whether the suppliers were using rough cut full dimension and planing the boards down or if they got their stock in standard widths. I would guess that all but the largest are stuck buying standard width.