Phosphate test kits only measure orthophosphates, whereas PR-10,000 reacts with all phosphate compounds.
If you test for phosphates, understand that you are only getting a reading of what that test kit can measure: orthophosphates. But orthophosphates are just the building block of more complex phosphate compounds. Those compounds, such as polyphosphates, phosphoric acids (and other products) may not show up on a test result.
So if you use PR-10,000 phosphate remover, it may still cloud the water. If it does cloud, that's because PR-10,000 is reacting with phosphate compounds that your test kit did not pick up.
The red cap test
In our opinion, phosphate test kits by themselves are not sufficient for knowing if you have phosphates in your water. A more reliable way to know is adding a small capful (~2 mL) of PR-10,000 to the water and seeing how much it clouds. No, it is not an exact measurement, but it will show if you have phosphates in the water or not. Below is a photo of one capful of PR-10,000 in water after about 30 seconds:
The red cap test is one of three water tests we teach that are 100% reliable, though not precise. The other two are the white bucket test and testing if CYA is present in water using a melamine reagent.