I guess originally that they planned to include the scenes but then decided later to take them out. But never adjusted the previews to account for that. That's the only thing I can come up with
Yes, that makes for fun watching! I like the previews, and I'm glad the DVDs have them. It's one of those now-forgotten features of TV shows, like (here I'm not referring to Columbo) the tail-end scene, a sort of one-minute encore that used to be common in comedies. They seemed made to be optional, so they could be removed for more commercials, although once in a while there'd be one that puts a whole new, final twist on the plot. Even the traditional opening theme song seems a relic now, for the most part.