ClearScore seems to have mad data?!

Danvers23

I found the rate on the MSE comparison tool, but then went to lender direct (Tesco) and did the eligibility calculator, and when it said there was a 95% chance I’d get the good rate, I applied. 

I’m all green on my credit file, but I do have quite a big mortgage already. But what I don’t understand is that I already had to put in my income and mortgage payments at the eligibility stage, so I don’t see what they could have found during the actual application that changes anything? 

Do you think it’s generally a better idea to apply to your existing bank?