I was reading about GAME Code in the Developer's Guide, and following its
advice I looked at the Pre-Game code for FRC. It appears that this preset
does not generate the 960 starting positions with equal probability. Of
the 960 possible positions, 216 have the white king starting on b1 or g1,
336 on c1 or f1, and 408 on d1 or e1. But the preset places the king on
these six squares with equal probability.
The easiest way to generate all 960 positions with equal probability is to
place the bishops first, then the queen and knights, and finally the king
and rooks.