What is the underlying reason why Stormtroopers miss their targets all the time?

It's even lampshaded in an episode in Mandalorian where the scoutrooper miss the mark somehow despite perfectly aiming his blaster to the target.I also heard that during the Death Star escape, (possibly) Darth Vader said that they should let them escape because he believes they will return to their rebel base, thus, leading the Death Star to Yavin 4, so the stormtroopers deliberately miss them for reasons (even Han pointed out the escape shouldn't be this easy, especially the likes of Death Star).In other scenes like the Battle of Endor, there is no reason for them to miss. Yet, in the intro in A New Hope, they wiped out all rebels inside the ship, with minimal casualties at best.Is there some sort of reason why for some reason, at some times, they miss their targets for some reason? and why they don't? Is it their blaster that they're using? The helmet they are using? Or is it sheer incompetence to their part? (despite being recalled as an elite soldiers). via /r/StarWars https://ift.tt/2De54y7