You would be correct if the ID were an integer being serially increased. I can sign up to your website today and get an ID X and then sign up again in a week and get ID Y, I can then calculate the number of new users you've had by performing Y-X.

If this ID is a timestamp then there's no such information I can get out of it from a small sample. I sign up today and get todays timestamp, then I sign up next week and get next weeks timestamp..?