Well I would guess that your grounds are on the same sheet or frame metal and are relatively close together and no corrosion on the connections or you've got some equipment that filters ground loops, if not than either your car's metal work was done with much care, you've got one souped up electrical system or you very lucky.

The truth is that all car audio equipment should have the same reference points in the car's electrical system both positive and negative but that tends to be difficult to do with out rewiring.