I got new medela bottles, and I'm still using some old ones that I had from Liam, and I just realized as I was transfering milk that the old bottles "3oz" is about a half ounce less than the new. So if I have 3oz of milk in the old bottle and I transfer it to a new bottle it only goes up to 2 1/2. WTF!? Does that make sense? How do I know which is correct?