Is this a case of people not understanding how 3.6.11 builds have been coming out before 3.6.10 RC3, etc.?
It's because people want to code in new features, but also want to have a version we* can release as the "official" FS2_Open 3.6.10, which doesn't have to worry about bugfixing the features that are just now getting added. Bugfixes from the Release Candidates (that's what RC stands for) will make it into the 3.6.11 builds, but the new features which haven't been bug-tested yet won't jeopardize the stability of the official 3.6.10 build.
*The SCP team which I'm not a member of, but I'm speaking in the first person plural for no particular reason.