Ok, I just ran the stupid checker 5 times.
Twice on my display, once your way, once my way. I got 0 ppb both times. That's useless.
Then I tried the fish QT tank...it's too high for the ULR. That's useless.
Then trying to figure out what I can do...hmm, maybe make some SW with tap water? Hmm... oh wait! I have a bin cycling some dry rock! So I test the bin twice, once my way, once your way.
Put the cuvette in the checker right after the 2 minute mix - 99 ppb
Continue to mix cuvette after the 2 min, and put in checker at 45 sec left on 3 min countdown - 103 ppb
So 4 ppb is within the accuracy and EMC deviation which is +/- 5 ppb (each!), so essentially, my readings are the same as I expected they would be.