yeah, thats how i feel sometimes :-). what do you mean with 15 sands of time? 15 times 80 sands of time?
i'm sure that they have no variability for any specific player. the only thing that came to my mind is, that they maybe realize when you play the same level over and over again, that the droprates seems to be more beside trash.
but i think that's my personal paranoia, i don't think that they have that implemented. when you play a lot you will get streaks when you don't earn anything special. the chances aren't so low:
lets say the droprate for premium in the dungeon is 20%. so 80% of the time you will get trash. the chance, that you will get 10 times trash in a row are 0.8^10 = 10.73%. So the chances are about 10% that you burn 80 sands of time and gain nothing good. it's a shame. especially because the chances for premiums could be even lower 15%-17%...