Whenever I read a US date I have the problem of figuring out what each number represents. So when I see: 10/11/12 I assume it means “10th November 2012” (which would be the norm for my country) or “12th November 2010” (which would make the most sense to me) but it actually means “11th October 2012”.
So, how did this come to be? Shouldn’t the order of elements be from largest to smallest or from smallest to largest? What’s more, one says hundred-twenty-two, not hundred-two-twenty.
It’s just how the language evolved in America. Look at it this way: 10/11/12 doesn’t ‘mean’ 11th October 2012, it means October 11th, 2012. This preserves the order of the shortened version. It just reflects the common American English way of speaking dates out loud.