If I had to guess I think it's probably has something to do with the way we say the date out loud. "September 17th" is shorter and sounds better than saying "The 17th of September" for a lot of people. So having the month go first on the month/day/year format sounds more like how a lot of people would verbally say it. Plus when some people think of a day in the past or one that is upcoming I think they think of the month the day is in first before they think of the day unless they are both in the same month.
The way it goes, Petra won't have to give out any t-shirt to someone who plays the game correctly, but rather 20 t-shirts to members reaching their 5,000th post...
{why don't they disable post count for this game...? }