1) You aren’t taking in account that many party political sites are also listed in other parts of directory, like here. If the submission strategy of one party is to submit all the sites only in the regional subcategories, you won’t see them in the number of the party category.

2) Of course much depends of the activity of the volunteer editors. Each of the “big three” party categories used to have its own editor, but as you can see from the web archive, the category of the Conservative Party haven’t had an editor for a while.

Some parties, like Lib Dems, have also made it more easy to find their local websites. If you look for instance the website of Lambeth Liberal Democrats, you’ll find a list of hundreds of other Lib Dem sites. Labour sites don’t offer anything alike, so they have made it much more difficult for ODP volunteer editors (to say nothing of other people with less experience and skills of websearch) to find their sites and to add in the directory. Of course it is also possible, that the local Labour branches have been more lazy than the local Lib Dem branches in putting up websites.