In our old flat I had a cheapo ten-quid set of scales which gave different readings in the kitchen and bathroom.
Chucked those out, moved house and just last week got a £70 pair (in the sale) and have the exact same problem! The bathroom has big enough tiles that all four feet can be on the same tile without any being in the dips, so I can't really understand why, when I rotate the scales 90 degrees or move the scales one tile over, the reading can change by up to 5lbs.
I've also tried them on the downstairs floor, which has admittedly seen better days but I thought it might be more solid, but no dice.
It's so frustrating! Obviously I might have just got a dud (and they're going straight back) but these ones had excellent reviews and nobody else seems to have this problem.
I was so looking forward to the Moment of Truth on Saturday after a very hard week's dieting, and still don't know how much I sodding weigh 
Is it the scales? The floor? Something else? Any ideas?