If yours and the doctor's both say the same, then I think those scales are correct, and the kmart ones are wrong!!! Don't get the kmart ones! Get ones that tell you the right number
I just got a garden variety mechanical scale from Big W for $11... because I broke my whiz-bang digital body fat etc etc scales in temper the other day - threw the damn thing!!! (it felt good seeing it break into pieces, but yeah, I couldn't revive it, so sad). Anyway, I think the mechanical ones should be more accurate, digital ones... don't know, I think they just spit out any old number they feel like!