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In many circuits the ground conductor to which the system is referenced is different than the return current conductor to which the circuit is referenced.  For example, consider a serial data bus such as USB that provides digital communications between two electronic devices that are remotely located but connected by a cable.  The USB driver circuit is mounted on a printed wiring board in a metal enclosure, e.g. personal computer housing.  The voltage on any one of the USB data lines is referenced, through the integrated circuits to which it is connected, to the power ground plane on the driver's printed wiring board.  The ground plane may or may not be connected to the metal enclosure.  The ground to which the system is referenced is the metal enclosure.  The ground to which the circuit is referenced is the printed wiring board ground plane.  Capacitance to Ground in this case is the stray capacitance between the printed wiring board ground plane and the metal enclosure.