This is my understanding. Might be a bit oversimplified but as far as I know it's more or less correct still:
There is phosphates in everything, it's not going to be possible to get away from phosphates. It is like other things however, including NO3, in that there is some uptake by organisms so there will be some minor fluctuations in the levels from one day to the next however it is usually a given that we tend to input more than a tank is able to uptake and thus there is a residual value which usually creeps up in the absence of a PO4 reduction mechanism (eg. algae, refugium, ULNS, GFO, kalk, etc.) Some phosphate is necessary for growth, although too high a value inhibits calcification and thus inhibits coral growth or encourages tissue recession.
Anyhow that said, phosphate is not just phosphate, there is inorganic phosphate and organic phosphate. Of interest to us is organic phosphate, but you can only test for inorganic phosphate. Generally speaking we assume that the level of organic phosphate is around the same as inorganic phosphate, but that may not always be the case.
I remember reading that if you boil a water sample for some period of time and then test for phosphate, you will get a different value for before and after, because the organic phosphate denatures into inorganic during boiling.
Having said that however, that may not be the explanation for the tapwater producing a zero value. Drinking water standards dictate acceptable levels for many different parameters and AFAIK, nitrates and phosphates are to be zero (or very close to zero), and municipalities are thus mandated to provide water as such. I remember there was a big thing in the news about some small town had to provide bottled water because the tapwater had a detectable level of nitrate.
I could be wrong but that's what I always thought anyhow.