"There, tests by Energy Department inspectors produced contradictory results, with one showing elevated radiation levels and another indicating normal levels. As a result, the ship was ordered back to sea for a definitive determination on whether any radioactive material was on board."
"But the official stressed that there is nothing to indicate anything more than background radiation at this point."
This seems to be easy to understand. A logical person would start with the assumption that two identical detectors gave different results. If so, the result is ambiguous until you determine which detector was malfunctioning. A false positive is less likely than a false negative, but it could go either way.
However there are different "flavors" of radiation. Detectors designed to detect neutrons are different than those designed to detect more common kinds of radiation such as beta and alpha emissions.
Since two detection methods are needed, it could mean neutron and gamma emissions were detected (detection method A), but no other kind of radiation (detection method B).
The story does not give the reason for the discrepancy. By the time the info got passed to an official spokesman, and then interpreted by a reporter, who knows what had been lost or garbled.
A dirty bomb could emit just about anything, depending on what was used to make it radioactive. A nuke, however, would emit neutrons but very little alpha or beta. Hence, a confirmed neutron emitter with no alpha or beta would get a lot of attention. But the spokesman and/or reporter might not know enough to understand the results.
Bottom line is that we simply do not have enough information to know what is going on. My bet is that they won't tell us anytime soon, either.