Death recap?

Anyone else added up the damage from the abilitys you saw killed you and be going wtf cause it only added up to 1200 health when you were full health at 1500-1600 health yet theirs a mystical gap that just magicly got filled causing them to get a free kill on you with no explanation?

I mean i could understand if comes from ap/armor pen or something but then shouldn't the damage counter pick that up as well and say the Q ability did X amount of damage instead of Y amount of damage but so far im not seeing an explanation in the death recap were the extra (10%) of the damage came from that killed me most of the time cause it shows the champions that hit me and their damage plus towers and minions that did as well, but still theirs that up in the air source of damage that did the extra (10%) thats not being explained.