My father, (who by profession was a CA with MBA, but is exceptionally handy) has regularly reminded me that walls/floors and ceilings are pretty much never straight and level, and over here they're brick and mortar, not wood.
This must be a well-known fact to all trades people who work on cupboards, tiling, door mounting, etc. But when you understand this, then you realize that everything is built to be forgiving of this reality.
E.g. prefabricated bedroom cupboards will always be fitted with fillers on each side and a kickboard for the bottom. This allows you to use feet/wedges underneath the cupboard to make it stand-up perfectly straight (which is not necessarily parallel with the floor and/or walls), but because of the fillers/kickboard being wide/tall enough and cut to fit the irregular/skew shape, you don't tend to notice.
Beading around wooden door frames is for the same reason, it hides the little gap that is invariably at points around it, either due to the hole in the wall being skew and/or slightly arched.
As I got a downvote I can only assume my tone came across as “how could you not know this?”, but my feeling when I wrote this was that it’s a bit of a funny and interesting anecdote the parent wrote and I’ve been similarly frustrated with how I easily make mistakes with seemingly simple tasks such as putting up a shelf, to which I have to laugh at myself about when thinking about it many years later.
I definitely prefer that with software it can be “perfect” and easily changed later if you find it’s not.
Gave an upvote to compensate :)