What does "one hit deaths" actually mean?

Solution 1:

As far as my own experience and other forums seem to be saying, one hit death does seem to be number of times friendlies have been one hit killed.

Therefore I can only assume that this was some form of bug or odd feature. I've found on one forum that apparently this sort of thing can happen at the end of the game when killing the boss:

"There's a bug at the end of the game, when the big bad boss MC's one of your guys. If you kill him, everything on his "Side" dies, so your MC'd character could of died also, but nothing became of it due to it initiating a cut-scene almost instantly, and then ending the game." - Vexx303