The Taliban had a bad year in 2008. Their losses were up, and the armed resistance by tribal militias to Taliban activities increased. The Taliban campaign against education continues to be enormously unpopular, as is the increasing number of civilians (especially children) killed by Taliban suicide and roadside bombings.
In an attempt to reverse their plunging reputation, the Taliban recently announced that they had killed 5,220 foreign troops last year (the actual number was 286). The Taliban also claimed to have shot down 31 aircraft last year (the actual number was zero). The Taliban also claimed to have destroyed 2,818 vehicles (that's about ten times the actual number) and killed 7,552 Afghan policemen and soldiers (again, about ten times the actual number.) The Taliban accused the Afghan, and foreign, governments of conducting a huge cover up by claiming much lower numbers.
To Westerners, these claims are absurd. In the West, there is a free press and free exchange of information. Hiding losses would be impossible. But to your average Afghan, a free press, and free access to information, is a rather foreign concept. So the conspiracy alleged by the Taliban has some credibility. Moreover, those who are leaning towards backing the Taliban, would be more inclined to believe the Taliban lies. The Taliban announcement was not meant to convince Westerners, but those people the Taliban already has some backing from. The Taliban want to encourage their followers and persuade those who are potential backers. So the Taliban claims make sense as an Information War tactics. Absurd data, but directed at an audience where the lies will have a positive effect.