When a Doctor or Hospital agrees to accept Medicare then that is what they are paid. The Dr or Hospital can bill you for the deductible or the difference between what Medicare allows and what Medicare pays, but the Dr or Hospital can not come to you for the difference between what they billed and what Medicare allows. This is the portion they "write off".
So if the hospital bill was 1000.00 and Medicare allows 800 and pays 700 of the 800 then the hospital can only bill for the 100 difference, not the difference between what they billed and what Medicate allows.