Piggyback - Comparison Sites?

Zoom7000

So, I'm guessing we all understand the concept of MVNOs (Tesco, Lebara, Voxi, ID, giffgaff etc) who piggyback off the main 4 networks (EE, Vodafone, O2 and Three)

I'm with EE and I'm trying to find a decent retentions deal. When I was getting them to try and match or better one of the EE deals on uswitch, they were telling me that anyone who comes from uswitch is also put on a piggyback plan, even though the contract is with EE. Vodafone sales tried telling me the very same thing. 

So, are they telling utter porkies here and just trying to get the direct sale, or is there truth to the deals you get from comparison sites just aren't as good as buying direct?

Cornucopia

Sounds like nonsense (though who knows what manoeuvres they may make internally in setting up your contract).  

Did they indicate what material difference it would make to your service, given that the main brand would still be on the contract?