For every setting except for 10 seconds, an additional +10 is displayed. For example, if 15 seconds is selected and you double-click, it shows +15 and immediately after that +10, even though only 15 seconds were skipped.

One more thing:
When the video is paused and I try to double-tap to skip 15 seconds, the seek bar doesn't move until I click play. Then it skips, but only once, no matter how many times I double-tapped while paused. I don't know if it's a bug or a feature.