Overscan and Underscan
Most television display manufacturers "overscan" the pictures on their displays (CRTs and PDPs, LCDs etc.), so that the effective on-screen picture may be reduced from 720 × 576(480) to 680 × 550(450), for example. The size of the invisible area somewhat depends on the display device. HD televisions do this as well, to a similar extent.
Computer displays including projectors generally do not overscan although many models (particularly CRT displays) allow it. CRT displays tend to be underscanned in stock configurations, to compensate the increasing distortions at the corners.
Read more about this topic: Display Resolution