Generally, the narrower a knowledge skill is, the more you know about a specific thing but the less you know outside of that narrow scope. Broader knowledge skills cover more area but more superficially.
For instance, you are prodding your mind for information about simsense star Foxy Fiona. If you have the knowledge skill "simsense stars: 1", you might know a bit of information about Fiona. IF you have the knowledge skill "Foxy Fiona: 1", you would probably know her short bio, sims she has been in, and at least some of the latest gossip about her.
On the other hand, while the person with the broader knowledge skill would know comparatively less, he would know these snippets of information about Foxy Fiona and other simstars. So if you were wracking your brain for what you know about simstar Buddy Fine, you would at least have some general information about him, while you would be mostly out of luck if you only had the knowledge of Foxy Fiona (maybe you would know that he starred in two of her films and dated her for about a month).