While you can argue if this is a bug or a feature, when using "--since " the first committer following that date will be attributed all the lines not changed after that date.
This is a consequence of the way git blame interprets the --since parameter, and perhaps makes sense in that context, but in the git-fame context I would expect lines not changed after the since to be ignored, or at least to have an option to get that count (lines changed after date by author).