I had noticed that as well. I wonder if it's because the lactose contributes to the process that makes it go bad, or because they treat it to make it last longer assuming it will move slower with a limited consumer base.

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Ah, a little googling, and it turns out it's a byproduct of the procedure to remove the lactose, Ultra Pasteurization (UP) or Ultra High Temperature (UHT) pasteurization. [1]

1: http://chemistry.stackexchange.com/questions/34884/why-does-...