I have chased many noise problems in audio and RF circuits caused by grounding issues over the years because of the situation you described and it is not a normal condition. There are internal circuit lows and there are system grounds, and both can become dirty, and sometimes when they come together, there is a bigger problem. The resulting noise can wipe out a receiver or sensitive control circuit in a heartbeat. Some low voltage DATA bus systems like CANBUS do not always use system ground for low reference for enhanced signal clarity and this may be what you are referring to.

Theoretically, there should not be any difference in potential with system ground throughout the entire circuit. In practice, any difference is accounted for in the design and is insignificant to the circuit. The ground on the fender should be the same as the ground on the frame and starter and battery, and any and every ground on the bike.

 

mr. zan makes valid points!
on non-ecu systems, ground is just ground. we are not talking about iffy connections, that is a given.
when we get into computer stuff, the game changes. there can be diff power sources. the biggest issue here is circulation currents and yep, the common ground is not your friend. many diff points can be ref'd to diff grounds isolated from one another. one issue that can arise is corrupted data, computers are stupidand the old saying, "garbage in=garbage out" is valid.

 

Y'all are correct. There are circuit lows and there are system grounds, and sometimes they are not the same potential. The circuit low in centralized computer controlled data-bus subsystems can be a positive or negative potential in reference to the electrical system ground. Sometimes system ground is a reference point for positive and negative pulses, and sometimes it is strictly isolated from the circuit altogether.