It is magnetic dip. If you want to avoid its effects, you need to fly with wings level and with no change in speed. If you are turning or changing speed, compass will lie.
Basically, in this part of the world (BoB map, I mean), needle will turn towards North if speed is increasing, and towards South, if speed is decreasing. Also, needle will lag behind actual direction while turning from south to West (or vice versa) through North, and will lead actual direction while turning through South.
In addition, there are bank and pitch limits, so don't look at compass while in break turn.
