Alright, thank you for humoring me. If I had a guess as to an explanation, here's my best guess.
1) Your low-range kit read it correctly and your nitrates probably really are at 6-8ppm. Since the limit of the low-range is 4ppm, the sample will look like its screaming hot.
2) You tested using the high-range dilution and potentially grabbed 15ml of tank water and 1ml of RO/DI water by accident. Just a guess here but only explanation I have for the super hot sample of 64ppm.
3) You then retested immediately and got 6-8ppm on the high-range which corroborates with the low-range test kit result.
Quick thought, sometimes it takes a bit for the nitrate level to register, at least for me it did when I was messing around with nitrate dosing as well. Call it a latent interaction with the biology of the tank but sometimes I would have a much higher nitrate reading a couple days after I dosed. Perhaps you are seeing something similar with the 6-8ppm of nitrate you are reading now?
Again, just all theories. I could totally be barking up the wrong tree.