What is happening on broadband comparison sites?

briley

I am looking to switch broadband supplier so have looked at a number of the comparison sites as well as quidco and MSE. I am getting two problems.

I see an offer, say quidco, with Shell for under £10 or Vodafone for £14.34 p.m. (after cashback and vouchers).  But when I click through I get told that my line does not support that package and what they will offer is 3 times the price - £30 to £40pm.

It appears that this is because the pole outside my house has fibre waiting to be connected to my house. So Openreach are going to give me fibre that with treble my broadband costs and I have no option in the matter.

Since I have already given my location and phone number to the comparison sites why don't they know this?

Secondly some of the sites make an offer that is flatly contradicted when you click through, as an example MSE says I can have Vodafone with a voucher for £150 - but the first page I reached (broadband genie) state the voucher is £75.