In some cases the mono mixes were the ones the Beatles themselves were present for. Some of the stereo mixes were the audio engineers experimenting on their own — this is why Beatles mono mixes are highly sought-after. Can’t recall whether Sgt. Pepper’s was a mono mix originally, but probably an easy search to find out — I’m too lazy to go to my bookshelf and flip through to the Beatles studio notes at the moment…
It was, and there are even some significant differences. On she's leaving home, the mono mix was speed up with the mono version's length coming in at 3:26 and the stereo at 3:35 [0]. I believe (running off of memory here too) that the first album where the stereo mix got some actual attention was the white album.