Sure, but that doesn't apply here. If my manager is happy with my work, by definition, expectations are being met, which means I'm entitled to be paid in full.

Sure, but that doesn't apply here. If my manager is happy with my work, by definition, expectations are being met, which means I'm entitled to be paid in full. Time and effort don't factor into the equation. The phrase in the offer letter works both ways: "hours... required to perform your job duties" can be more or less than 40.