Stardust Crusaders: Jotaro Kujo vs High Priestess(.gif)

On the title:

I wish I knew better about shmups. I remember noticing some TASes/proplays and noticing how the patterns were actually more lenient than what I used to play.

Answer: Dynamic difficulty. Some really old and deceptively simple games had that mechanic in place already in one form or another (Chief Wigwam/Scalpem in Sunset Riders for example had different patterns for no credit loss and no life loss).

So reading on I learned that a buttload of variables, including how long you hold/push fire button will only aggravate the mobs, and certain power-ups will also pour fire ants in the CPU's crotch, so a "proplayer" will simply avoid unneeded fights, power-ups, and shoot just enough so they get consistent and avoidable patterns.

And then there's scoreplayers, one time I was even directed to a readout by Raiden devs on how they tackled the scoreplayer meta.