There are a few users who say otherwise . There are many who say there is a subtle difference and some like you who say they are not alike at all. I will take your word on it because I've never heard them.
I had both during a short period of time and imho HE-500 is a better headphone, with better tonality, deeper bass and more extended and cleaner treble. Even the soundstage on HE-500 is more open, wider and deeper.