gps on road bike
speed sensor on mtn bike.
I’m not convinced it’s gps quality due to tree canopy. I think it’s just the frequency of the pings vs. the direction changes on a mountain bike. The computer tends to draw straight lines between points. On the road or a relatively “straight” course, those dots are pretty accurate. When you are going up switchbacks on a mountain bike, that line may miss the part where you actually turned around.