If a 5 years old child ask his father how a plane is flying, the father will explain him as a certitude how he think he fly but it's more or less false. Indeed do you know that nobody knows how a plane fly ?
if you don't trust me, check this link :
In IT field, we entered some years ago in a very fast world where framework in a lot of different languages are pushing every day the limit of "the possible to do in a short time" for any developer with a basic level.
The plane question is then the question that a lot of project manager need to answer to people "they are managing" (i hate this term in any case but..).
What was true some months ago, are false now because the last update for the last framework X do the trick now faster then before and then the choice done 2 years ago is not the right choice now for this project even if there is a terrific mood around this new wave of technos.
"Classic" Project Manager Job in IT field is becoming more and more useless because a young developer fan of a new techno may prototype now 10x time faster .
Then the skill to "empower" people will become the core value of IT management more than a strict expert view which is becoming less and less possible to do.