I can only guess. The mono mixes were the definitive mixes, so there should be no differences in subsequent releases. However, early stereo records (not just the Beatles') were mixed to show off that the record was indeed stereo, and that the buyer did not waste the extra dollar purchasing the stereo version of any given album. There was a greater amount of difference between channels then than there was in later years.
I suspect that the later stereo releases of Beatle albums sounded closer to the mono than the original stereo LP releases.