I'm may get some flak about this from the contributors to those two topics but I hope the majority will understand why it has been done:
When looking for 'unread posts since last visit' - as in the link in the user box at the top, the results contain every post made since then. Due to the prolificity of those two threads it makes it difficult for those not interested in Dross or Jokes to find posts more relevant to Fuerteventura.
By giving Jokes and Dross their own category users can take advantage of the 'Unread Posts' link that each category has to the right of the Category title. This will then show unread posts for that category only.