The labels are basically arbitrary, but I believe that there is a logic behind the way the labels are laid out. If I remember correctly, the designations are arranged to insure that the loads are distributed evenly between the two power transformers. In other words, all three "left" channels (front, side surround, and rear surround) are on one transformer, all three "right" channels are on the other, and the center is tossed in on one of them.

The 770 and 7200 are the same in many regards, but I don't know if there are some aspects of the 7700/7500 design that influenced the internal layout of the 7200 - could be that the way the taps on the transformers run to the amp channels changed between the 770 and the 7700. Certainly the way the 7200 is labeled would fit with the logic I describe if the transformers are wired directly to the nearest amp channels...