$96 sends "The overcharge on transaction 677348921 has been refunded to purchaser 2475244. If you have any futher questions please dial our busy call center at our convenience. <beep>."
$100 sends "Hi, we overcharged you how much? Nintety six dollars? Here, call it a hundred, keep the change. We weren't trying to skim six months interest on $96 from you maliciously and we're human and take a personal interest in your problem caused by our company and by us, we're not faceless and mechanical, we have kept your money but we've paid it back and more, have the small benefit of $4 and please consider giving us the benefit of the doubt and dealing with us again in future".
Exactly- we do the same thing when people ask for refunds of our iPhone app. It's only $1.99 so if they go to the trouble of providing their PayPal, we send them back $5. Half the time they return it and take another look at the application.
In general, most people have such terrible experiences with customer service that they go in expecting a negative experience, placing them on the offensive. If you're responsive, helpful, and personable then they'll flip right around and tell their all friends about how awesome you are.
Not because $4 is fair compensation for her trouble. Or because $4 is so much money that she should be happy.
But because they actually took a moment to consider what message a $100 refund sent to the customer vs what $96 would send.