I don't know the math of your shader, but when you look through the upper left corner of your screen and see a point in a distance of 10000, its depth coordinate is about 8000, dependent on the angle of the frustum. That's just Pythagoras. If the frustum angle were 180 degrees, the depth coordinate would be zero.

The far plane of a frustum is a plane, not a sphere. A7 treated it like a sphere. Only at the screen center the depth coordinate in your example would be indeed 10000.