I have noticed this as well, my usual situation is with the small fire spitters:
SENARIO 1: Run up to it, hit it twice, knocks me back, but i get to hit it once more before being out of range, then run back and hit it twice, then it dies. Total hits to kill 5
SENARIO 2: run up to it, hit it twice, knocks me back but not as far as usual, so hit it twice its dead. Total hits to kill 4
I dont know why sometimes it knocks me a shorter distance than other times but i think it has something to do with when i junp and being in the air and stuff.
(All testings done with the basher)