bdp, yes, the later Beatles mixes can be very different between stereo and mono. Listen to the stereo version of the song Yellow Submarine and you don't hear "a life of ease--everyone of of us" and on the stereo Sgt. Pepper's Reprise you don't hear McCartney's "barker" shouting at the end, right before the segue to A Day In the Life. Somebody fell asleep at the board on those. But the early Beatles recordings are not that way. As I said, the mono mixes of those were (according to George Martin) made from the stereo mix. He made the stereo mix the way he did to facilitate the making of the mono mix.
The Beach Boys mixes were a bit weird anyway, in concept, because you've got an artist with only one good ear trying to mix in stereo. Doesn't work so well. Brian did assist with the 90's Pet Sounds stereo remix (which I prefer to any other mix of that work).