I didn't say it ignores categories -- it knows which questions go together in a category, and learns what to use for a given category as it sees question-answer pairs for it. What I said was that it ignores category titles.

However as it happened I was wrong about this; slight misremembrance, sorry. Watson does note category titles, it just doesn't weight them very highly. Apparently it learned this automatically during its training games. Source: http://www-03.ibm.com/innovation/us/watson/related-content/toronto.html