http://www.stage6.com/user/bas_vp/video/1937380/HG10-Pinacle-Splice-Test

So my video, which I ran for 56 minutes, and got about 18 minutes per segment, which was at least 75% pitch black, still didn't compress further to say 30 minutes per segment?

Where can I learn about why this isn't happening with avhcd?

AVCHD is ALREADY a compressed format, and supposedly pretty well optimized... you're not going to "re-compress" beyond a point. Even though in theory it should take less space to say "the whole frame is pitch black", there's still probably overhead that limits how much it can compress and still maintain the data stream as intended.