It's not even a promised delay.
Some forum-goer said "I hope there's no delays" after seeing the trailer, j. Bergman said "don't worry, it'll definitely be out the 20th or 21st" and people took that line and flipped a [censored].
He probably just said it because the three companies have a habit of releasing the games at 11pm-2am for Europe, so TECHNICALLY it doesn't come on the 20th.
As for who delays the most, PS3 has delayed releases for days at a time before for various reasons, Steam just generally takes their time getting off their asses and flipping the switch, so it comes out ON the date but late for Europeans (so usually 11pm-2am for europe), and X-box is generally fairly reliable, releasing anywhere from 6pm to 10pm.