Terrible GPS Accuracy - What is going on with my Fenix 5?

I recently went hiking in Yosemite with my girlfriend. I bought this very expensive watch for the explicit purpose of doing hikes with it and instead found my GF's vivosport wrist band to outperform my watch on GPS accuracy. See for yourself:

It's not even a contest. While the vivosport had a small hiccup around mile 7, mine for some reason though I was rocket jumping all over the place on my way back from the waterfall. In all, the watch added almost an extra 5 miles to what was a 10 mile hike.

When I bought this watch, I thought it was mountain-compatible. I am very disappointed for lots of reasons, most importantly because I wanted to capture our great adventure but instead got a bunch of gibberish data.

Is anyone else experiencing these issues? Have I done something incorrectly?