Maybe they do it without realizing. The recruiter's goal is to fill the job as soon as possible. I even think they feel it is in their best interest that the candidate be qualified, so I'm not trying to knock recruiters.
Aren't they better off presenting 3 candidates, but one clearly stands out? The last thing they want from their client is a need to extend the interview process because they can't decide. If the client doesn't like any of them, you just bring on your next good candidate. This way they hedge their bet a little.
Any experience, insight or ever heard of a head-hunter admit this? Does it make sense? There has to be a reason why the choose such unqualified people. I've seen jobs posted that clearly state they want someone with a CS degree and the recruiter doesn't take it literally. I don't have a CS degree or Java experience and still they think I'm a possible fit.
Edit: I'm asking this question because there are many posts that assume recruiters cannot recognize talent, but maybe it is not in their interest to recognize too many talented candidates and make it harder for clients to decide. Eliminate the confusion. There is one candidate. Make that person an offer. It's a win, win, win.
Maybe this falls under the theory of Too Many Choices?