more formula in bottle after mixing ?

K • Mommy to Ava and Kieran 💕
I measured out 6 oz of water, placed the 3 scoops of formula, and when I shook it up and the foam settled there was 7oz in the bottle..how does this happen? I know it's not foam and I'm really good at making sure there's exactly 6oz of water in the bottle (I get paranoid about giving her extra water lol)