Some companies prey on people with less than perfect credit, and they get offers, too. The person is probably the type that pays, but pays alot of interest and creates good margins for the lender, with a risk of default the lender feels is acceptable, since the lender is shopping for consumers in the sub-prime market.
Most pre-approved lists are lists of consumers who have, at the time the list was generates, criteria that meet the requirements for the credit card offer, the equitly line or whatever is being offered. Minor changes in the lag between the generation of the list and the person’s actual credit would result in a pre-approved offer leading to a declination of credit.
Sometime, the declination of credit might be based on additional information gleaned from the actuall application/process of activiation. For example, a pre-approved equitly line might be dependent upon thr am’t borrow and a certain income or ability to verify addresses, etc. Failures in any area might lead to a declination of credit, even though it started out as pre-approved.
This hardy covers all the scenarios, but should give you a flavor for how it works.
Lets say one might see a guy in a BMW and pre-approve him to receive a free BMW certified oil change. When he pulls into the shop, one sees he has an aftermarket tuned engine and so is declined to him the free BMW oil change. Sure, he was pre-approved, but only based on a non-scratch-the-surface look.