Agency ask this question "Have you being interview by any agency"

I have went to couple of agency such as Robert Half, Teksystem,etc and pretty much all agency ask me this question. I am little surprise when they ask me this question. I don't know why they want to know but in my mind, I get kind of suspicious. and sometime I don't know what to reply them.

Some of my suspicious are:
for example, I went to Teksystem and got interview. for some reason, Teksystem might think I am not very professional or some kind of criteria they think I am not a good fit. They will ask me "have you interview by anybody or by any agency?" Sometime, I say this is my first time.

Now, I go to Robert Half and they ask me the same question "have you interview by anybody or by any agency?", If I answer yes, I went to Teksystem.

Does Robert Half call Teksystem about me, what do they think about me? if Teksystem say "he is not very professional, etc, etc, something bad", then Robert Half will say, ok he is not that great, not professionally so I won't make contact with him or something like that. Do agency talk to each other about their candidate?

or they want to know you went to all these agency and you couldn't get a job so they might think he is not a good candidate.

I thought they should focus on you to find a job and not focus what agency you went.

I am just kind of new to the contract agency and I am curios.

There are a couple of reasons why they ask - and neither has anything to do with getting you a job or assessing your qualifications.

They are fishing for leads. For expample, if you were interviewed at TEKSystems, they want to figure out where TEK is doing business and then will try to steal it, and vice-versa.