Led Zep '71 BBC sessions - balance tweaking?

Last night I was listening to a BMG double cd of the live BBC sessions that Led played from '68 - '71. CD Two is all from '71. On the first six tracks, I found myself tweaking the balance 4db to the left! I almost never play with balance, but for this recording, I just had to. Also, for a couple of the later tracks, I tweaked the balance 2db to the left. Is my hearing funny, room problems, bad left recording mike, etc? Does anyone else have this album and experience? Not a hi fi pressing, just over-the-counter stuff.