1) Presumably the county has a much more accurate test than out hobby grade kits. Our kits are toys in comparison.
You could try diluting the sample with 1 part tap water and 3 parts RO
water, so its a more readable color on the test kit, and then multiple the test kit result by 4, just to see how different the reading is. Kind of pointless experiment though since its still dependent on the accuracy of a hobby kit.
2) Maybe the guy diluted the sample himself. Not saying he did, just saying. Perhaps give the county a sample yourself?
In the end, how about growing some Anacharis, Hornwort, or other low-light easy plants? They'll grow fast, and trimming them is an excellent form of nutrient export.