Yup, it's true -- they just use a different pasturization process.
I was curious about this myself a while back and looked it up.
It's something like all milk could be pasteurized so it lasts that long (ultra pasteurized?), but either it's a bit more expensive or they think consumers would be slightly weirded out by the longer dates, so most "regular" milk doesn't use it.
As to why the organic milks are often ultra pasteurized, it's because the market for organic is smaller and the milk is more expensive. Stores aren't buying the stuff in huge quantities and throwing it out when it gets near the expiration date. They buy small batches of it and it sells slowly, so they need longer dates. It's possible in some areas it's comign from farther away (since there aren't a ton of organic dairy farms around) so the long shipping times also necessitate better pasteurization.
as a side note, there are some organic brands that sell UHT shelf-stable milk that doesn't need to be refrigerated until it's opened. So cool!