Supersonic

I took this with my GF1 attached to the Tamron mirror lens. It is probably as decent a shot as the one I took with my Canon. I'm increasingly drawn towards ditching all my Canon gear and getting a GH2. Not that Canon are rubbish - rather the Panasonics are pretty useful.
What I find interesting is that there is about 3 stops difference between the two shots but if I were to guess (the moon is always going to be the same light value surely - even if this one was take from the 'side'.) I'd have said a couple of stops max. So are Canon over-claiming on ISO value - or are Panasonic under-claiming?