On the left, the app thinks I went 3.8 miles, but on the right, the course plotted on a map is 2.4. That's 42% extra. It also just about doubled the distance between my bus stop and office, though I can't plot that one on a map because I cut through a field and can't make the map do that. But I think it's about half a mile in real life.
I do like the MapMyRun site, though. It's much easier to plot on there than on plain Google Maps. Many thanks to porkchopblog for the tip! Hopefully there will eventually be a feature where you can correct the saved run to get rid of the extra zig-zagging they think you do. Makes me wonder about FitBit stats. Any FitBit users know if there are similar issues?