Estimates never represented a promise in the first place. If you have someone who is holding you to your estimates, you have to address that.
Regarding difficulty, easy things aren't even expected to be faster than hard things. I'd rate a backflip as much harder than counting to 100,000, even though it wouldn't take nearly as long.
You can't promise you can beat a game on hard 2x as fast as you can on normal, or 3x as easy.