Why is flash TTL metering independent from ambient light metering?

In other words, why is ambient exposure metering independent from TTL metering? Let's say Im taking a picture in a room that has a window and some daylight is getting in,and Im shutting in P (program mode)If the camera exposes for the available ambient light when I press the shutter release button half way, then any flash light will be an excess of light, because the camera has already calculated the shutter speed or aperture needed for a proper exposure