The math might be correct it just doesn't reflect what it says. Its 20min saved compared to going all of the roads at the speed limit all the time.

So every time you slow down (because of trafic, a red light, a crossroads or for any other reason) the app will calculate you as going slower and "loosing" time. Of course you couldn't actually have gone the speed limit in those situations but the app can't know that. It actually can't calculate the time you saved.

But I can't find the app on fdrois so I can't check.