Match completion does depict the rate of quitting a match before ending it and or dcing and not getting back before the victory screen. Match completion is not rounded up, but rounded down. However, to give players a second chance and for having more accurate, recent, completion rates - the match count for the completion resets every so often. I believe it is 30 days but don't catch me on that. That is why you would appear to have 100% when in reality the number is different - because for the current time period the game checks, you have 100% of your matches completed.