The Beatles never considered the STEREO mixes as the main mixes, presumably because the technology was relatively new and given the 4 track format they were working on at the time, decisions had to be made to be putting all vocals on one side or other such tough choices.
To really properly hear the songs up through Sgt. Peppers, try to seek about the MONO versions. Might be tough to do on CD...may have to go to vinyl for that.