it is possible that you could be testing during mid-cycle.
Are you finding that this is happening quite often or just this one time. I, myself, once had this happen. Tested 2 hours later and everything was back to 0.
How cycled or mature is your tank?
Lunchbox ... "i'm a man, but i can change, if i have to, i guess"
Tankless ... for now ... until i find what i really want (and what the wife lets me get!)