Actually, this has been bothering McCartney for a while. I heard an interview with him a year or two ago (on Fresh Air) where he talked about a trip to Italy he'd taken. He was some place where a musician was playing 'Yesterday' on the piano. Later, after the musician had left, McCartney flipped through the song book on the piano and found that in it 'Yesterday' was credited to Lennon only. Apparently, the publisher had decided to abbreviate the credit so he dropped the second name, McCartney, from it entirely. This incident, as well as other thoughts, helped shape his decision to claim more credit for his work.