Although I cannot see any micro-bubbles present, even when using a 10X jewelers loupe, at this point it is my best guess as to the cause of the issues as it's the only thing that could seemingly explain the results obtained.

Today I altered the process. Instead of inserting C2 into the checker when the 3 minute timer starts, I continued to gently mix the sample until ~2:45. Then rolled it horizontally in an effort to remove any micro-bubbles that night be adhering to the sides of the cuvette. The results appeared more consistent but unfortunately (or fortunately depending on the perspective) my PO4 levels have dropped to the point virtually all of the results now fall within the stated accuracy claims of the device....

5/04/12 7:00AM Initial - 07 ppb. Immediate follow up tests - 0, 4, 4, 3, 0 (agitated/rolled during 3 minute timer)
5/03/12 7:25AM 7 min - 04 ppb. Immediate follow up tests - 3, 1, 2, 6, 1
5/03/12 7:00AM Initial - 12 ppb. Immediate Follow up tests - 1, 0, 1, 3, 1
5/02/12 7:00AM Initial - 12 ppb. Immediate follow up tests - 3, 0, 0, 0, 0 (fresh GFO)
4/30/12 7:00AM Initial - 25 ppb. Immediate follow up tests - 3, 12, 10, 4
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

I'll have to wait until the current batch of GFO exhausts, then repeat a few tests to confirm.