I would suggest that you look at this on very long time Scale. What can often be seen in Biofilm based waste water treatment plants is that the Biofilm Matrix (EPS) act as a buffer for Phosphorous compounds. I.e if you stop FeCl3 dosing to increase P to the biology it can take up till 2 weeks before you start to see P in the effluent.

Because in the biofilm there are more and more bacteria than in plantonic water medium

Since the biofilms are fixed in attached processes and not as disturbed by turbulent mixing in the suspended processes, the fixed biofilms could provide longer periods of contact with the waste waters and in turn contribute to removal of most of the phosphates