IT’S NOT ABOUT GRADIENT … forget it at all …
… IT’S ABOUT YOUR VIRTUAL GEARING 
Probably the best explanation here … (just my humble opinion)
btw … I have set my difficulty at 100%, … but just because of I have my trainer bike equipped with 52/36 x 11/32 gearing … if I have another gearing, I will set my trainer difficulty to different-more reasonable level …