Sometimes the TiVo guide data is incomplete as well. If that happens then the TiVo will record them all just in case as it can't tell if you've seen it before.

I've just had a look and it's only got the 3 specials on Discovery. The guide data is only for 2 weeks in advanced though so I don't know what happens after the 28th which is the last showing of the 3rd special.