Inaccurate AP nitrate test?

• pH:
• Temp:
• KH:
• GH:
• NH4:
• NO2:
• NO3: 2
• PO4:
• Iron:
• Dosing: 3.75 ppm NO3 (morning), 10 ppm NO3 (evening), 1 ppm PO4
• NO3:PO4 Cumulative Dosing Ratio:

Notes:
There was a noticeable increase in algae (drk green) today. I had added an extra dose of phosphates yesterday to see if it made any difference, so it seems that it must have affected algae levels. I received the new Seachem phosphate test and a Seachem nitrate test. I tried out the new nitrate test, and found to my surprise that the nitrate levels read near zero (~2 ppm). That 2 ppm reading was taken in the evening after the morning dose of 3.75 ppm nitrates, so that means the nitrate levels must have been running at zero these past several weeks. I double checked the Seachem nitrate test against a reference nitrate sample which comes with the test. The old Aquarium Pharmaceuticals nitrate test that I had used until now read between 10-20 ppm. If the levels are more closer to zero, that could finally explain why the plants have been stunted and algae levels have been increasing these past weeks, despite my increasing the levels of all nutrients except nitrates.

50% water change:
25 mL Amquel+
2.5 tsp baking soda
3 tsp Equilibrium

After the water change this evening, I promptly added 10 ppm NO3.