Honestly, a change of 80ppb is likely in the noise for a home test kit, it's not much. For reference, that 10mL, when grown to a very high density (a live culture, not a concentrate), takes approximately 0.013 mL of Guillard f/2 fertilizer (both parts combined). A standard f/2 formula uses 5g/L worth of phosphate, so 13 uL of that contains roughly 2.6 micrograms of dry phosphate.
A microgram of a nutrient added to a liter of water raises is by 1 ppb, so if adding around 3 micrograms of a nutrient to a tank makes it raise by 80 ppb, your tank is about 37.5 mL in volume - a little more than an ounce. If my rough calculations are off by a factor of ten thousand, that puts you in a 100G total system volume ballpark.... but they're probably not that far off.
The reality is like what I said above, dosing phytoplankton doesn't lower your nutrients by anything unless you have a massive amount growing (a visible tint to the water is the right ballpark), and the organisms eating it lock up the phosphate temporarily in their tissues for growth, eventually releasing it back into the water column and increasing the phosphate. I've been dosing phyto heavily to my tanks for more than a year and they ALWAYS read high for phosphates than without - above 1 ppm is quite common, I've seen almost 3 ppm when I get lax with the phosphate remover. And those levels are achieved with 300 mL of phytoplankton dosed daily in the long term to an ~40G system, and the phyto I've been growing is denser than any culture I've bought commercially.
