Ruminations of a meandering mind

One observation that I have carried with me for long is that doctors very rarely stick to the time that they have given for an appointment. For the past two months circumstances forced me to visit a lot of doctors and not one of them came on time. This is something that I find very hard to discern. What could be the reason for this? Is it because they are not unable to judge the time that they require to treat one patient and this changes their schedule? Even in the cases where I have been the first patient to be treated on that day, I have found that the doctors never arrive at their scheduled start time. Maybe some of my doctor friends can throw light on this.