That's interesting. The variations could simply be down to the testers accuracy?If you are using their low range phosphate checker, most of the data falls within the accuracy range of the checker. The phosphate levels are something like 0.05 ppm +/- .04 (.01 to 0.09). Great numbers in my book!
From hanna’s site:
Accuracy @ 25°C/77°F ±0.04 ppm ±4% of reading
I use the 'regular' Hanna Phosphate checker. HI 713.