I think the holes in the bottom are drawing up water...
I make feed up fresh, put into pyrex with water and ice for speedy cooling. Of course when you shake the bottle some froth appears making it look like theres perhaps half an ounce or so more. But on more than one occasion theres been what looks like up to two ounces more in there!
Anyone found this with these bottles? I dont use the feeds when this happens in case they are contaminated/not made up to proper ratios.