My Killing Spree performing 4 attacks instead of 5

As title, I checked today my KS by recording it and playing in 0,25 x speed. After recording and checking it few times its obvious to me, that its performing 4 attacks / 8 damage packets, something like:

2080 + 1592
3990 + 2991
1848 + 1493
1987 + 1249

or

1088 + 2848
1909 + 1217
3946 + 2624
3567 + 2787

even tough it stated in description: " attacking an enemy every .5 secs with both weapons until 5 assaults are made" So the question is: is it a bug (which seems to be quite impossible after all this years) or am I missing sth?