Hi,
... because they have different meanings.
a battery operated system usually has a signal_GND. This is where each signal refers to. But it may also have a shielding, this is chassis_GND. Chassis_GND may be connected with signal_GND, but not at a random point.. it will be connected to signal:GND in a way that Chassis_GND currents have low impact on signal_GND and it´s reference to the signals.
A mains operated device with metal case will have Earth_GND. It´s for your life protection. It may be directely connected to signal_GND, but is may be connected with any device in sereis, like a capacitor or a resistor.
One is more for safety, one is for operation, one is for noise suppression....
Klaus