The mechanic is likely making up this story, since he likely has a number of full bottles of oil an most cars do not take an even number of bottles.
He is supposed to fill it to the factory designated level, and give you whatever is left over in the bottle to take home. But that takes work, and some thinking so he does not bother.
As others say, 1/2 quart is no big deal in a Taurus engine, 3/4 of a quart leaves me puzzled. He would have been better to UNDERFILL it by 1/4 of a quart and charge you less.
If he overfills out of the bulk tank, he is really out to lunch (cars usually don't consume much oil), unless he is trying to charge for an extra quart. I would go to another mechanic.