Hmm, as you are not 'at' the turn yet (i.e. the white line is going to the closest point in the route) we're simply using the difference between the incoming and outgoing edges of the turn. If the white line were to point at the turn, it would use the difference between the currently reported GPS heading and the outgoing edge.

The only way I can explain this is if the actual route data is going past the turn and then approaches it from the other side.