I think that's because Brian said in the Summer Days liner notes that it's the first one Carl "dug singin'" - therefore he didn't enjoy singing Summertime Blues, Pom Pom Playgirl, Louie Louie or All Dressed Up, but he sang them anyway. I would imagine it's the Beatlesque nature of GDTM that made him dig singin' it...
That...and the fact that Leaf and co. got the vocal credits wrong on the orig. two-fer CD liner notes. But you're right Carl never considered himself a "singer" until '65 or so, before that his focus was on the guitar.