"lol x-stats" became extremely popular during the 2005 Summer Contest. Amazing Telephone originally wrote it as an offshoot of "lol internet", and UltimaterializerX picked it up and ran with it; not surprisingly this led to its widespread use across Board 8. Unfortunately, its meaning has been changed in its popularity... when Ulti began using it, he was saddened by the predictability of current contests - how it's not about who will win, it's by what % they'll win that we worry about. Eventually, when matches like Magus vs. Knuckles and Mario vs. Samus led to results that did not agree with the 2004 x-stats, the phrase was coined and used for anyone who makes statements regarding any contest using x-stats to back their words up.
People took the phrase to mean "x-stats are wrong sometimes!" and now post it incessantly every time there is an upset in a contest. The phrase is most often used by people who blindly followed the x-stats and want to blame the failure of their bracket on something else.