Drop rate buffs and Mirrorworld hearts

Hello there. Again, I have a question about rng or drop rate or, perhaps, a bug...

Last days I was doing Mirrorworld and the quantity of hearts obtained and drop chance does not have a minimal correlation.

With 3 pets (+20% each), maid potion (+75%) and npc buff (+100%), still getting ~15-23. I'm not sure if is a normal behaviour but a guildie with maid potion + npc buff getting 61 hearts where I got 23. So with +60% drop chance 38 hearts less and doing the same instance the same day. This is an example because I have watched it few times.

Could be something which could cause drop chance not adding properly? Is just persistent bad luck?

Wait what? I didn't know that tiny detail. Now I'm not sure about that... I remember it was less than 1 hour difference between his try and mine but could be that and in some point we were in party.

I'll check it, thank you very much.