As for why you failed the polygraph, it is likely that you reacted more strongly to the relevant question about selling drugs than to the corresponding "control" question, which may have been something like, "Did you ever lie to someone in a position of authority?" See Chapter 3 of The Lie Behind the Lie Detector for a more detailed explanation of the dubious assumptions on which polygraphy is based.

That you were administered a "yes test" is some indication that you may have also been suspected of countermeasures. You'll find the "yes test" explained in the message thread Counter-countermeasure Techniques.

At this point, I think it would be prudent to appeal the results in writing, requesting a re-test. In addition, if your polygraph examination was with a federal agency, you might wish to contact Mark Zaid, an attorney representing several plaintiffs who are suing the federal government over its pre-employment polygraph policies. Mr. Zaid's phone number is (202) 223-9050. You'll find filings from these lawsuits here.