Set "Repository triggers the build" but builds still happen automatically on every branch create

I want a scenario where builds are automatically started when a (Git) commit is pushed to master, but I don't want any automatic builds when commits are pushed to branchs. But I want auto-branch creation so developers don't have to create the branches themselves.

So, I've set auto-create branches to be true, and I changed the build trigger to "Repository triggers the build when changes are committed". I've added the Git post-receive trigger so that it checks each commit and only sends a build request to Bamboo if the commit goes on the master branch. This all seems to work fine and my builds do get run properly when I commit to master.

However, I ALSO appear to get builds running when a branch is first created/pushed to the Git repository. This is very annoying because we use a lot of branches and building them when they are created, rather than waiting for something interesting and correct to be pushed to the branch, means we have a lot of worthless builds.

It doesn't seem right to me that newly-created branches are built immediately, and it especially doesn't seem right if I've set the trigger type to "Repository triggers the build", since my triggers did NOT start a build in this case.