Part of it is test, part of it is how PO4 reacts in the tank. It always binds and unbinds from rock, sand and other surfaces and it tries to reaches equilibrium with the water.I agree with your points. I was just pointing out that the swings happen based on ways we test for it.
So best time to measure is once the equilibrium is reached. Also I try to collect water from same location and away from my tang bathroom location. Those buggers are particular where they do their business…
I used to chase my tail as well, I even had 4 different PO4 test kits…. Randy finally set me straight…