I have different expectations depending on whether you worked 10 or 40 hours. The _reality_ is that you and your employer need to come to an agreement on expectations.
If I can meet your 40 hours expectations in 10 hours, then what?
You adjust your expectations upwards
You want my maximum efficiency for what you think you're paying me for, which is 40 hours
But if I know I'm going to do 40 hours minimum either way, what is my incentive to work harder and get more done?
"That's your job, it's what I pay you for, it is your contract" you say.
But you pay me either way and as long as you're happy with my output then it doesn't matter if I actually worked 10 hours or 80