Back in the day, I used to work for the other lot - Equifax. In league with the devil imho but it paid well. And I could walk to work. So every cloud etc. I digress.
The biggest factor in personal credit rating (then, it may have changed) was being on the electoral roll, and having an address that could be verified and for over x months/years - the longer the better. Payment histories (credit cards, loan repayments, household bills, etc) would be used only where those details were provided by the companies involved - and not every company did. So you could have an abysmal payment history with some company or other and the credit reference agencies may not know about it, so it wouldn't feature as part of the rating calc. I'm guessing Experian worked similarly. The exact credit rating calculation was a bit black box in truth but the rating that Equifax/Experian calculated and provided (to credit card companies, lenders etc) may or may not be used by them in deciding whether to give you a loan/credit card/whatever.
Edit: oh, and sharing an address with someone (or having the same address as someone, a former resident for example) who has a bad credit rating could have an effect on your rating too - but you can get a disassociation by speaking to the whichever credit reference co.