A lot of organic milk sold in grocery stores is actually reconstituted powdered milk from other countries where they have larger organic dairy operations. We just don't have the infrastructure here, though it's getting better. As a result of the pasteurization and then dessication process there is very little in the milk that can spoil until you introduce something into it. It's possible that your 1/2 gallon is reconstituted dried and the gallon is fresh.
Re: why does the gallon of org milk have longer shelf life than half gallon?
probably this.
This. different brands have different amounts of pasteurization.