I've seen the same thing. Sometimes you get them, sometimes not. This can be hours apart or even within a few minutes of each other.

I think it's either that...

...the spammers aren't always online.
...they can only do so many search terms at a time, or have certain words they key in on.
...Limewire doesn't always return all possible search results (maybe I don't have access to all possible nodes, etc).