The reason to me seems simple. If you had an 933 G4 ibook, the G4 powerbook would really be in trouble.
I assume most peole just look at the processor (now that the chips are all G4's on the portables)
Knowing nothing they see 800 vs 1000 they think welll that is a big difference...
933 vs 1000, you've got a problem.
I assume apple has the ibooks totally aimed at the average joe, but just in case for people like me and others who want the best bang for the buck....
the 12 ibook competes with the 12 powerbook and the 14inch choices compete with the 15 powerbook.
933 vs 1.25....
It makes sense.
In reality, they could have made the 12 inch 1ghz... and it still would be good for some people to buy the powerbook, but apple figured, that would be too close and not give the average user the difference factor.