my Wii fit does the same. What really offends me though is that even though my BMI is only a fraction above the absolute ideal of 22, it has given my Mii a fat arse! I am a bit pear shaped but how the hell does the Wii know that?!
The Wii fit is highly inaccurate, and IMO (and from what I've read) shouldn't really be used by those under about 14? It gives false information and I'm sure I read on here about someone's dc feeling rather upset by what it told her.
My mii is a blob, but I'm going to the gym now and using proper scales to weigh myself. I'll go back to Wii Fit if I need a laugh.