Formula Amount

So I don't mix a batch of formula for my son each day (I make it for each feed), and I have run into the observation that when making a 2oz or 4oz bottle after adding the formula it pops up another 0.5oz. Now I get the idea of displacement, but wouldn't it still be 2.5oz or 4.5oz? And it's not bubbles as I stir in the formula until it dissolves. So my question is, when I note how much he's eating, is it still 2oz when it comes up as 2.5oz on the bottle (I use Dr. Brown's) or is it only 2oz. Now it might not seem like a big deal, but those 0.5oz do add up over the course of the day and he unfortunately went from eating 27oz a day (when I was pumping/feeding since I had the exact amount) to now eating ~21oz a day as I was putting he only was eating 2oz or 4oz a feeding even when the bottle showed 2.5oz or 4.5oz.