I didn't know they did that for lower bitrates. Then again, I don't think I have any music with a bitrate lower than 92kbps so what do I know . I would imagine they lowered max volume to save your ears from the horrendous compression artifacts that would surely bombard your eardrums while listening at full volume.
I am inclined to believe that that is merely serendipity. If you use something like LAME and encode the same AIFF or WAV to 256kbps and to 24kbps I doubt the amplitude would differ much if at all.