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Many trucks have a frame that supports the body.
The body is attached to the frame by rubber mounts, so there can be a lot of motion between the body and the frame. The master cylinder is attached to the body while some of the other brake components are attached to the frame. Also, when you step on the brake pedal, you flex the firewall of your car, moving the master cylinder but not the other components in the brakes. A car is not always built with perfect accuracy. On one car the distance from the master cylinder to the anti-lock brake system pump will be slightly different from that on another.