What is the make of the phosphate test kit? I have acutually used a phosphate kit for aquarium use (powder reagent) to test my pool and the results were identical to the results from the pool store. They all basically use the same reagent that turns blue if phospates are present. The difference would be in the comparator used. Printed ones on cards are usually not quite as accurate as either plastic or liquid tube comparators. Then again the phospate test for a pool does not need that much accuracy. You just need to get to 0 ppb (or, at least, under 200 ppb) phospate if you are going to treat for it. It is entirely possible that your tap water has high phospate levels so testing tap water would not tell you if the kit was accurate or not.

Phospate kits measure in ppb (parts per billion) not ppm, usually.

My question is, why are you worried about phosphates? If you do not have an algae bloom that you cannot kill it really isn't important. If you do have a resistant algae bloom then removing the phosphates from the water will help only if phosphates are the 'limiting factor' in the algae growth. If there are also high levels of nitrates in the water removing phosphates will have little or any effect at controlling the algae since nitrates are also a favorite algae food! (the amount of carbon dioxide in the water also has an effect on algae growth...this is influenced by a combination of 2 factors: your pH and kH--carbonate hardness or total alkalinity)

If your fill water has high phosphate levels as your test indicated and there are no nitrates in the water also then treating for phosphate would be effective. You might want to do a check on your fill water for nitrate also.