The numbers seem incredible to me - I have not kept track of all the flights I have made when there was a mechanical delay but my estimate would be in the 90% - 95% range.
You have to know what they base this on. Does readiness mean any defect or only a defect that causes a cancellation. A delay of 10minutes, 20, 60, 120, where is the line drawn. I can tell you with 100% certainty that it is not a cut off at the scheduled time of departure or it would be no where near 99.7%. Also many defects are carried for a few days and after a specific time period they would cause the aircraft to be grounded if not fixed. How do they take account of these?
Modern airliners are amazingly reliable considering their sophistication. I often get in one that has been flying for days with only an hour or there abouts on the ground to change passengers and crews every 10 hours. Without a doubt the ones where I work spend far longer in the air than they do on the ground throughout their life.