I'm not sure will my answer help you in any way but I indeed have noticed that your sub will actually drift few degrees due to sea state or wind (or maybe even hard coded bug with somewhat "realistic" outcome), especially if you travel in a strait line for a long time...
This means that there is indeed one more extra variable in the equation but I can not tell for sure what is it for sure...
Check this screenshot I made a year ago or so...
EDIT:
Of course, I was talking about SH5 with latest TWoS v1.6.08 on board. Not for Stock SH5 with only few mods in it...