Its not double. The problem is the use of the word nitrate interchangeably with Nitrogen. The two words do NOT have the same meaning and yet get used interchangeably.
Your drinking water is required to have a measure of Nitrogen no more than 10ppm. That is measured by measuring the nitrates and doing the Math to covert it back to Nitrogen.
Roughly 45ppm of nitrate is equal to 10 ppm of nitrogen. Which means your tap water is 1/2 the standard, not double.
This website from the state of California does the best job of explaining the confusion I have seen, and even this is not really straight forward.
http://www.cdph.ca.gov/certlic/drinkingwater/Pages/Nitrate.aspx