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.

I'm convinced I gain if I'm sitting too long even if I'm not eating. I believe the gravity theory.

So I went out and bought a new scale, BEFORE reading all these comments, LOL! And it ended up being junky. It was $7 and the last one left on the shelf and the box was slightly damaged...but I was feigning for a scale and needed a fix regardless.
It was crumby and rickety, and I'm returning it tomorrow and I'm going to make a truce with my current scale.

This is what receipts are for. Take it back. Trust the current scale and make nice-nice to it. It will eventually be your friend, it just takes a lot of patience.