Every living thing contains Phosphates. Because trees were once alive, the carbon contains Phosphates. It's not a good idea to get cheap when it comes to carbon. Yes, most carbon is coming from most of the same sources but carbon is customised based on application and price level.
It's a good idea to soak your carbon in RO
water for a couple of days before adding it to the tank. The RO
water will act like a magnet and pull most of the Phosphates out of the carbon. I see people all over the boards saying that GAC
absorbs Phosphates and eventually re-releases them. This is not true. The Phosphates were there from the get-go and the leaching is the worst when the carbon is brand new not after it has been in the tank for a while.
Most carbons for aquaria use are no longer washed in phosphoric acid. However, the more expensive acid washed ones typically contain less Phosphates.