Eliana, I'm convinced on the gravity thing too. I make sure to weigh myself as soon as I get out of bed, after peeing, just because of this. haha
I think your scales work fine, mine do the exact same thing. I get different weights here and there, so I make sure I get one weight three times in a row, and that's my weight that day.
Journey Started: June 22, 2010 (233 lbs)
Met Goal: June 22, 2011 (133 lbs)
Starting again April 16, 2015 - losing weight from 3rd baby!!