In order to stay off the list of airlines that have the most delays and late arrivals, some airlines are now padding how they report flight times. By over estimating flight times most flights arrive early. Even when people know the flight times should be shorter than reported, most people still appreciate showing up artificially early. Zappos employs a similar tactic when you choose 3 - 5 days shipping, but are given free next day delivery. Exceeding expectations beats meeting expectations.
On the flip side, people don't like to be disappointed. One of the easiest ways to disappoint people is by making a commitment you can not keep. This is why most infomercial products fade away so quickly. Their primary purpose is to get you to buy the product only one time. Often they will give you the second one for free. The product never lives up to the hype, you will be disappointed and never buy from them again.
Our nature is to talk a big game and work hard to match our story, but unfortunately we too often fall short. Instead, by under promising and over delivering, we avoid disappointment and create appreciation.