From what I understand is there is a seperate flag for first run episodes that is in the guide data, but isn't visible to us anywhere on the screen. This first run flag is sometimes set wrong. It might be that an old show has the first run flag set even though the episode is an old one and it records something you've already seen, or sometimes it could be that the first run flag is not set even though it is an episode and you miss a recording you should have got.
This is especially annoying with TV shows from other countries. Sometimes the show airs first in another country before it airs in the US. Tribune sometimes uses the original air date in the other country in the guide data. That really doesn't matter though as long as they set the first run flag correctly, but sadly they don't do that sometimes. For example I think it was either Holmes on Holmes, or Holmes Inspection that was particularly bad about this. The shows originally aired in Canada a while back but just started airing on HGTV. Tribune would often put the original Canadian air date as the OAD, but then play around with the first run settings. Sometimes the shows would correctly be set as first run since they had never been aired on TV in the US before, but other times they wouldn't set the first run flag and the episode would be skipped. It was really frustrating and people had to just set the DVRs to record both old and new episodes to be sure they got everything.