First, nice job! The fit on those looks quite good, despite the gap.
I've had a few issues lead to base-line gaps.
1) Cutting below the line. This is easily the problem I have most often. Especially on the edge of the board, it's easy to cut in just a little too deep, which means there will be a gap.
2) Not cutting square across. Is there a gap on the inside? If not, you angled the cut a little bit.
There are others, but they don't actually make sense here. On this board, I'd check to see if there's a gap inside. If so, you cut below the line. If not, you didn't make the cut square across the tail board.