This was brought to my attention by someone else on the board. As found on Big Soccer.....
The percentiles are constructed from the sample of "veteran refs" (guys w/ at least 50 games experience).
G = Games
Y = Yellow Cards
SY = Second Yellow Card resulting in a Red Card
R = Red Card
PK = Penalty Kick
?/G = Per Game
Original link here
http://www.bigsoccer.com/forum/showthread.php?t=1775283