The seating position and keyboard is centered on the screen, the mouse is off to the right. I've never seen anybody try to center their mouse on the screen- then you'd either be bending your arm inward/across your body or sitting left of center of the screen
When using the standard WASD+mouse arrangement, you don't exactly want to center anything, but rather put the WASD keys at the left hand's natural resting place, and likewise with the mouse and right hand. With many large keyboards, the mouse then ends up colliding with the keyboard, so you have to spread out your arms. With a small keyboard this isn't an issue.