Perhaps its related to empty space, meaning the more tunes (=lower bitrate) the more empty space for the CPU to rest (=more battery life). That makes sense. Otherwise I can't see how 'wall to wall' music encoded to 270 bps would be any more efficient than 384 bps except that you wouldn't get all the music at 384 as you would at 270. This all assumes that the other parameters are the same.
I'll take your guys word for it because I'm pretty much a neophyte here but it still doesn't make much sense. No matter, the number of tunes one can fit on a given volume probably trumps most other issues anyhow for portable use. I think its worth figuring out the smallest bitrate one can stand...especially with correction files where you can always get back to square one.
peace
dog