I wonder if that’s at least partly because it’s John playing the leads on “Get Back” and George the rhythm?Preston's parts and the strings are pretty much always in the rears, but guitar panning does seem to vary a bit throughout the album. On "Get Back", I hear the rhythm guitar mostly through right rear and the lead maybe 75% in left front. Then on "I Me Mine", the lead fills during the chorus come entirely from the left rear and the acoustic is floating somewhere between the right channels.
and of course “I Me Mine” was recorded 14 months later with John not present. No surprise that one would have a different mix.