it's all about the instructor and the agency. The agencies set minimum standards to cover their butt from lawsuits. Instructors may change that.
I.e. I require a 10 minute tread, 200m swim, 25m underwater swim, and retrieval of a 10lb brick from the dive well before I accept someone into a basic OW class. Later, they learn to scull with fins on *basically treading water with fins but no arms allowed*, then have to complete a 800m circuit with 25m u/w swim, 375 surface swim, while treading water don mask/fins/snorkel, 25m u/w kick, 375 surface kick, and a bunch of other stuff that has nothing to do with diving directly, but everything to do with in water comfort.
The agencies want to make it easy,