I honestly don't know exactly how the MCU considers the inputs relative to any azimuth (a misuse of the term azimuth, since
there's no real relationship to any headings in the strict sense of the term) but my best guess would be that when you're "setting
the reference point" it zeroes the stack and then it likely considers how many pulses are sent for driving in whatever given direction.
By tracking the sum of whatever it's counting then using simple math it then knows how much of that same input to apply to equal it's
intended RTH direction. There needn't be any relationship to compass or gyroscopic directions at all other than it's simply keeping a
running total so it knows how much to add to that in order to send it back. My use of "degrees" for rotation was only for ease of explaining
how the effect works.
It IS because there's no compass or inertial navigation guidance (gyroscope) referencing that most RTH functions are crude "ballpark only"
functions. They don't compensate for drifting or wind so, while it's better than nothing, the best they do is at least send the quad back in
the general direction it came from. The ones that DO employ compass, gyro, or GPS correction are much more accurate of course, but
the "toys" just don't do that.
