I'm not sure if there's a rule per se, but the placement changes the emphasis. Let's take a sample sentence like "I am hungry". If you want to emphasize being hungry you would say "I am really hungry". If, however, you want to emphasize that you truly are hungry and aren't/weren't lying, then you would say "I really am hungry". A less common way to do the same thing would be to put the "really" at the very end of the sentence, but with a slight pause before it; "I am hungry, really". This is more colloquial though and looks strange written down.
Your particular example is a strange one in which both would imply essentially the same thing, but you're right. Yours is more common.