Baby Clothes Sizes

How can baby sizes be so far off sometimes? We'll get clothes for Jack that are supposed to be whatever month he's in and they're not even close to fitting him. Do some of these baby clothing companies just sort of take a guess as to what sizes babies are? I think they may be using Cabbage Patch Kids for models because when they're off, they're really off. Usually jumbo gigantic or too small for a a baby hamster. Not usually too much in between. Granted, most of Jack's clothes are hand me downs so they've been through the wash a few times but that shouldn't matter too much. It's a mystery. Thank God he has two thousand outfits to pick from.