I do not have heat pumps as you may know. Here is a GUESS on it...Seeing heat pumps are used normally for maintaining heat and not as the main heat, that is why the gas heaters came on. Once it gets to X temp, it maybe uses the heat pumps to try to keep that temp.
Again, I am guessing but really sounds like it could be a good guess. Then again...Maybe I am thinking these things are smarter than they are. LOL
I love trying to figure these things out...even more so when their are no manuals.