It is because they keep track of 30 games, if #1 is a Finish and the #30 is a finish and say games 10-15 are quits, then after you play one more then #1 goes away (it was a finish) numbers 10-15 go to 9-14 and the new game is #30. So, you still have 5 quits out of 30 and the % doesn't change. It is only when those quits get pushed out by new games will you % change.
NOTE: You started with 30 games and the 1st 3 were labeled 'quits' so you had 27/30 = 90% to start. Then if you played 3 games and didn't quit it went to 100%. From then on quits will lower your % but it won't go up until those quits get pushed through the 30 games.