Going from 16:9 to 4:3 is not stretching, its compressing. The TV signal doesn't 'ask' what the resolution of the TV is, you just get what you're sent. The computer on the other hand does check with the device to see what the resolution of the display is, and only wants to send what the display is set up to handle. Because it reports a 4:3 resolution, the computer only wants to send that - despite the fact the TV is displaying it in a 16:9 shape. Normal displays computers are designed to work with use symmetrical pixels (square or round), not the rectangular ones your display uses.