I forgot to stop recording
It happens to the best of us. You get into your car to drive home and you forget to stop Slopes, so now you've got an extra "run" on the highway with a top speed that's screwing up your stats. It's an easy fix though!
First of all, tap on + /- Edit Runs:
This will take you to the Timeline Editor where you have full control over the recorded data. From here you can easily add, remove, or tweak a lift or run.
So for example, tap on the "run" when you started to drive away and hit delete. You'll be prompted if you want to delete this, plus delete all lifts/runs after or before that point.
Once you're done, hit the checkmark in the top right hand corner of the screen which will take you to the main editing screen. From here, just tap on Save and you're good to go.