We tracked down and fixed a killfeed bug that could credit you with a kill you didn't make.
If an infected or animal killed another infected, animal, or player while you happened to be standing nearby, the system sometimes guessed you were the killer and added it to your stats. That's fixed now - kills are only credited to whoever (or whatever) actually landed the blow.
This affects leaderboard totals and kill history going forward. Past entries aren't retroactively changed, but new kills are now tracked correctly across all three servers.