I hate that too... according to my wii fit im obese and i hate seeing the mii change from slim to fat after a body test haha. Guess it gives me a little nudge to bring my bmi down. My bathroom scales are also different but as long as the wii is calculating how much im losing (or putting on) accurately its not too bad.
I also hate that, and then the Mii looks so sad
My balance board is never the same as my bathroom scale. i don't take much note of the measurement tho as i go on what my bathroom scale tells me