The big issue is when a manager knows the difficulty of the task but not the context it's being done in. A project may be perfectly reasonable to complete in 4 weeks if it's given the priority it deserves, but I know that I'm almost certainly going to get pulled off to do something else so it's going to wind up taking 12 weeks, and then with a very moderate 33% padding giving an overall length of time of 16 weeks, the manager (who has no visibility to the thing which will pull me away) thinks I'm adding 300% padding. Then they say "surely you can do it in less time if we just don't let you get pulled away" and of course you say "well I've been pulled away from all of the past 27 projects over the last 5 years" and they say "don't worry I'll make sure this time is different."

It's not a lack of technical competence, it's a lack of introspection and managerial soft skills.