An ESTA can be denied and there are checks in place. The approval process can sometimes take a day or more. So it's not just checking if you are eligible, they

An ESTA can be denied and there are checks in place. The approval process can sometimes take a day or more. So it's not just checking if you are eligible, they also determine if they assume that you would be granted entry to the country. Since it's valid for 2 years there can obviously be changes after you've been granted the ESTA.