There were several levels where my "enemies killed" was just about half of total when reaching the exit portal with nothing more to kill to be found. Not small numbers either, it was in the ballpark of 500/900.
Are there some really MASSIVE secret areas I missed? I never went for all secrets with some levels really overstaying their welcome already.
Has it something to do with playing on hard, maybe counting the monsters of the other difficulties into the total?