Git repository

I'm trying to fully understand the history as presented by git. One of the things I can't work out is this:

Take a look at src/pike_memory.c on the 7.8 branch in gitk. For the last few commits it says 7.7 and 7.8 branches only, which is what I expect. But at the commit "Added DMALLOC_USE_HASHBASE mode" it suddenly claims 7.4 and 7.6 too, and that that commit precedes both tags v7.4.512 and v7.7.40. Why is that? I can't see this commit or any descendant on the 7.4 branch.

All commits after that one are also claimed to be on essentially all branches. This is something I see in general all over the repository. If the history of several branches is viewed at once then this makes it difficult to see on which branch(es) a commit is really made.