Too much branches after mirroring a repo

Summary

Today I cleaned my automatic mirrored repo (there were a lot of branches) but after the next mirroring the deleted branches get synchronized again.

https://gitlab.com/maxraab/gitlab-ce/branches = 1637 branches

https://gitlab.com/gitlab-org/gitlab-ce/branches = 557 branches

Expected behavior

1/3 of my current branches ..

Actual behavior

I got a lot of already merged branches.

Did I misunderstood this feature?!

/cc @smcgivern @stanhu