My day job is a computer engineer, so I am quite particular about details and precision. This is one of the reasons I was so disturbed about the different results from the different tests.

I was re-reading Taylor's test kit instructions last night and realized I was doing one thing wrong. I was taking my samples from near the surface of the water (about 3-4" below the surface) instead of the recommended 18" deep. I took the samples at "elbow-depth" and observed the same results, but at least this rules out one possible source of variation.