Ah phooey, it sounds like heaps of babies do it, and there aren't THAT many bandy legged people out there! River stands all the time too, always has, he just loves to use every possible muscle in his body at once! It's one of the only ways I can burp him too, I'm only just getting him to sit without arching his back!
The only thing I've heard about holding babies under the armpits is that you shouldn't carry them like that too often cos it might be bad for their shoulders - you should hold them a bit lower down to take pressure off. And I don't even know if that's true.
Good on you for having a strong baby!