Extra means more. If you ask for something extra spicy, it should come spicier than mere spicy would be. When I order a burger with extra pickles, it should be made with more pickles than they usually put on one.
So, logically, extraordinary should indicate the thing being described is even more ordinary than what would be ordinarily described as ordinary. Something so predictable that it is inconceivable that anyone could be surprised by it.
Yet, what it actually means, of course, is something that is beyond ordinary, more than ordinary. Here "extra" causes the term being modified to become less of what it was rather than more.
There is no learning English. Stop trying.