I suppose when the Beatles were learning songs they would have to buy a copy of the record, play it on what was probably a poor quality turntable, through small speakers, and try to work out the lyrics for themselves. They didn't have the luxury of CD-quality recordings, decent headphones, and lyric websites to help them.
So I'm sure there will be quite a few mistakes in the lyrics of their various cover versions. And songs will have been passed from band to band so the mistakes will have multiplied, like in a game of Chinese Whispers.
Plus John was notorious for forgetting words to songs - even his own - so he may have just made something up on the spot to fill in a gap.
In fact, it's surprising they got any of the words right at all