Originally Posted by aswracing

Compression ratio is only half the compression formula. Cam timing is the other half.

You can only build compression when both valves are closed. Well, as the motor starts it's compression stroke, the intake valve is still hanging open. So the piston starts coming up, but the intake valve is still open for awhile, and no compression is building.

The reason for later intake close points is high rpm torque, which is better known as "power". The idea is to keep filling the cylinder as the rpm rises. Cylinder fill keeps happening during the early stages of the compression stroke because the incoming fuel has inertia.