Media Buying: People don't remember signing up

I've had a weird couple of weeks now and i was wondering if someone could shed some light on what is going on.

So i wanted to test out media buying, put $5k aside for the project and signed up with 5 companies.

I did CPM popunders and have got plenty of leads from them HOWEVER....

I keep coming across clients that when you ring them they have absolutely no idea that they signed up to the page and ask us "why did you choose me" or "where did you get my details from" or "who is BRAND NAME" etc....

Its staggering how many people say this and checked in with my other sales guys and they are all experiencing the same thing...

Someone told me this is Co Registration which i believe means that the person signed up to another offer a while back and then a bot fills in the data for them on other sites...

Thing is, is there is no incentive for the media companies to do this as we pay per impression, not per signup..... The only reason i can think of is that you think you're doing really well with leads and increase spend but surely its a short lived strategy..

If anyone could shed some light on this i would REALLY appreciate it!

I think its co-reg. You still need to get some sign ups in order to keep you spending money with them (even if you are paying per impression).