My understanding of the talent is the number of points you spend is an approximate number of procs per minute you will get.
4:50 @ 5 per minute = About 24 procs expected. You got lucky and had 31.
4:39 @ 4 per minute = About 19 procs expected. You got unlucky and only had 15.
From wowhead comments.
That's just the RNG at work. One time you got really lucky and had extra procs. One time you simply got a little unlucky. That's just how random numbers work.. . . for 5/5 KM you average 5 KM procs.
. . . a weapon with 3.0 speed (20 melee swings per min) will have a 1/4 chance to proc KM each hit.
. . . a weapon with 1.0 speed (60 melee swings per mint) will have a 1/12 chance to proc KM each hit.