Because to most project managers the problem always looks easier during the spec phase than it actually is. Very few people can groc the problem before doing it

Because to most project managers the problem always looks easier during the spec phase than it actually is. Very few people can groc the problem before doing it. Occasionally you over-estimate, but usually you are under. That's why a good manager of project managers usually just slaps a 25% increase on any estimate (adjusting for his knowledge of the particular PM's abilities).