That's because it was harvested and the organic die off on the rock contributed to the po4. The same thing would happen if you pulled LR out of your tank, let it dry out, and then reintroduced it into a system. Doesn't mean it's bad rock, just has to be handled with care.

You could always go with reef saver which is not supposed to have any organics on it since it is mined. I went with fiji which is also harvested so I decided to treat with LC. LC bonds to the po4 molecules thus allowing you to pull most of the po4 before using the rock. You're not necessarily treating the rock with the LC, you are treating the water it's in to get rid of the po4.