The PO4 removers work by precipitating lanthanum phosphate particles. But the phosphate is not actually REMOVED from your pool, till those particles are filtered out, and backwashed (or washed, with cartridge filters) away.

I don't know enough about the phosphate test, to know whether it would register phosphate still present in the pool as unfiltered lanthanum phosphate particles.

I would treat PO4 removal as a week-long process: add the PO4 remover, filter and clean normally, and then -- after a week -- check your PO4 level. You might also want to retest the fill water when you do - PO4 levels in drinking water can vary, depending on what's happening at the treatment plant!