Nitrate test - wait time?

My nitrate test kit says to wait five minutes before comparing the
colour against the chart. I assume usually this would be for the
colour to develop and if anything, show a higher reading (and
stabilise). However I've noticed that after the five minutes I get a
lower reading.
Eg last night I did a test and the reading immediately after adding
reagent 3 was around 10ppm. Five minutes later it read closer to 5ppm.
Last week when I did a test the immediate reading was 20ppm, after
five minutes 10ppm, and after a couple of hours (I forgot to rinse the
vial) read 0-5ppm.

I'm assuming I should go with the first (highest) reading, but has
anyone else experienced this?

Cheers,
Stu