You never know if it's going to be the last, next to last or second to last person that's going to get the actual cam. I can't count the number of times I've gotten the last kill in a match and it'll roll back to 10 seconds earlier to show someone else getting a kill.
We used to think maybe it's the total number of points that person got with the kill, but it'll ignore a double C4 on two attackers for a single gun kill (or ignore a double gun kill on two attackers for a single betty kill).
I wonder this also. There has been a couple of times that I'd get a triple kill with a pistol for the final kill... but nope, Mr. slow reflex/lmg/TF/backofourspawn/camper gets the final kill cam. Weird how they couldn't get the kll cams right in this game.
It has happened to me as well at least two or three times. Maybe something to do with scorestreaks? This one time in KC someone get a random gun kill and just before the game ended I called a LS which got a kill and we won right after but it still showed only his kill and not mine, which was last.