Well, they're just reflecting the reality of the delays in the system. They're not trying to goose their on-time statistics. It's too expensive a proposition. The more extra time the pad into the schedule the fewer flights the existing equipment can make (in other words, poor aircraft utilization), and so revenue ultimately suffers. Which basically means that the horrendous state of the US air traffic control system is in fact hurting the airlines profitability and likely keeping fares artificially high.