It shouldn't at all.
If you have loose skin it just makes it easier to find a bit to pinch. If you don't have loose skin, they'll pull it till it pinches then measure that.
Skin isn't fat. I mean, skin is not thick, not wide, not something that'll measure on skin calipers. Skin is thin, even if it's loose skin.
So when they pinch, they're pinching your skin (and fat if you have any) and measuring how thick that is. Loose skin just makes it easier to find something to pinch. Doesn't come up 'thicker' than skin they've pulled together for a pinch.