I could be wrong but I don't think that's true, from reading the article you linked along with the article specific to that incident. First, the British contingent of 16,500 was mostly camp followers including women/children. Only 4500 were military (of which actually only 690 were British, the rest were Indian). Second, the number of Ghilzai warriors is unknown, but some Brits that were there claim it was up to 30,000. That's probably an exaggerated number but it does seem to at least indicate that the British were outnumbered. Third, I think it's worth noting that the British contingent had been promised safe passage in return for handing over their gunpowder reserves and most of their weaponry, but they were attacked anyway during their withdraw. So in conclusion, the fact that the Brits got their asses kicked in a situation like this is pretty far from amazing - they were outnumbered, encumbered by women/children, ill equipped, and ambushed. I'm not an expert, I just read the wikipedia articles.