That isn’t how the development works. Let me try to explain:
There are two active development branches, “6.x” and the service release candidate; currently 6.7
6.7 was created when 6.6 was released.
When 6.6 was released, then the the current state of V6 is “frozen” and designated 6.7.
All new changes go into 6.x.
The Service Release Candidate is left alone for a month and only major defects are fixed. The idea is to get a stabilized branch of the code.
When the SRC branch is stable, it is released and everything changes to the next increment.
The “incomplete” you describe is the process that happens in the Work-In-Progress builds that will produce the next Version of Rhino.
Does that help?