So much for that theory...

Today I made sure to mix C2 for the full 2 minutes. The good news was despite the 2 minute auto time-off period detailed in the instructions, the checker did not turn off. At the end of the 2 minute period, virtually all of the reagent was dissolved. The test results were virtually identical to the previous,

4/27/12 7:00AM Initial - 17 ppb. Immediate follow-up tests - 4, 2, 7, 0, 6
4/26/12 7:00AM Initial - 16 ppb. Immediate follow up tests - 0, 4, 6, 5, 4
4/25/12 7:00AM Initial - 18 ppb. Immediate follow up tests - 2, 0, 0, 0, 4, 3
4/24/12 7:00AM Initial - 29 ppb. Immediate follow up tests - 11, 9, 8, 12
4/23/12 7:00AM Initial - 43 ppb. Immediate follow up tests - 5, 8, 9, 13, 8

This coupled with the following response from Hanna provided in another thread seems to do away with my theory that undissolved reagent allowed to sit in the still cuvette for 3 minutes might have caused the variance...If the powder still isn't completely dissolved by the end of the 1 minute 45 second shaking, it will not affect the reading as long as you see that it is dissolved when you remove the vial after the three minute countdown and reading. If there is still undissolved powder at that point, it is likely that you will get a false low, but it is impossible to quantify.

I'm at a total loss in understanding why these results vary so widely. I believe my test methods have been sound including alternating the cuvettes used for C1 and C2 on subsequent days to account for any optical variance in the glass - especially since the results have been consistent and reproducible. I'm disappointed that despite responding to other topcs Hanna has seemingly ignored this one.