Originally Posted by Jeff Lee

Some pump gas, as is sold here in arizona for example, are "oxygenated".
From the EPA:
Oxygenates are fuel additives that contain oxygen, usually in the form of alcohol or ether. Oxygenates can enhance fuel combustion and thereby reduce exhaust emissions. Some oxygenates also boost gasoline octane.

The Clean Air Act requires use of oxygenated gasoline in areas where winter time carbon monoxide levels exceed federal air quality standards. Without oxygenated gasoline, carbon monoxide emissions from gasoline-fueled vehicles tend to increase in cold weather. Winter oxygenated gasoline programs are implemented by the states. The links below provide information about oxygenated gasoline, winter oxygenated gasoline areas, oxygenates (such as ethanol and MTBE), and health effects testing of oxygenates.

Basically pump gas that has been oxygenated has qualities that are similar to those banned fuels unacceptable to NHRA. Since field testing is not able to decipher the exact recipe's used in the fuel being tested, if it a fuel check reveals certain properties, it's no good.
Now, proper field testing is a whole 'nother story.