You make it to the hallway before you tear open the envelope. You count it twice. Sixty five bucks?!? Where's the rest? Surely there's been some mistake. You go back into the room but she's disappeared, having taken her lab coat and clipboard with her. She never even said what sort of research she was doing. Now you're upset, but there's nobody to complain to. You go home grumpy.
Now let's run that scenario again, but instead of $100, the envelope says $40 on the front. You still end up with $65, but instead this time you're up $25. You're elated, happy with the way things turned out.
Dipping into reality for a second, the stock market and real estate bubbles seemed fun while they lasted, but when they collapsed, a lot of people with high expectations were severely disappointed.
The higher your expectations, the more you open yourself up to potential disappointment. If, on the other hand, your expectations are low, you might be pleasantly surprised. As a customer, pleasant surprises are much more conducive to happiness than expecting the best and being disappointed. To be a happy customer, expect less.
To flip that around in a business setting; give customers a little more than they expect. They'll be pleasantly surprised and maybe they'll tell a friend or two. Letting customers down, or otherwise not quite living up to expectations is a recipe for dissatisfied customers.
It used to be that a typical dissatisfied customer would tell ten friends, which is about eight more than they would tell if they were happy with a service. Nowadays any customer can project their discontent to the whole world on a blog like openrevolution.ca for example. Keep the customers happy. To that end, it's much better to underpromise and overdeliver.