Hey Dale, thanks so much for trying my app out.
So the data that is provided is from Mapbox directly and I definitely wouldn't rely on it 100%, especially the terrain data, which is going to be inaccurate and likely stale in most cases. It's mainly provided in there as a general guideline so quoting the figures to anyone who expects accuracy would be a no-no, in my opinion.
You're definitely not doing something wrong, it's essentially just tied to Mapbox's database. I will add this info to the updated app description!
Thanks for the feedback.