has a lot to do with which organic standards they are adhering to. the new fda standard says that it can be labeled organic if there's 90% organic ingredients, and labeled as having organic ingredients if it's 75% organic - I'm remembering this, so the figures may be a little different. You do have to be cautious what you're getting if it says Organic. The Cal Organic standards were the highest and any organic outlet can use this classification, but it does require certification. The new FDA standards are about the same as California's, but I'm not sure which parts differ.
Also, for small farms that are just naturally organic, sometimes they can't say they're oraganic because they cannot get the proper ceritifcation. But, certification or not, smaller farms can proclaim they are organic naturally. Getting to know these sellers is a good idea because then you can trust yourself to know what you're getting.
I'm referring to general organic guidlines, not for milk explicitly. But, the guidelines do include these standards.