Just to be clear the voltage on the negative side is zero right?
It's all relative, I'm afraid. The Neg terminal is usually assumed to be at zero potential just to make us humans confortable. However, the POS terminal can also be a reference point. If you reverse the leads on your voltmeter and place the terminals on the battery posts, the meter reads -12-ish volts right? That is because you just changed the reference over to the POS terminal. The meter is just telling you that one probe is at a more negative potential than the other and displays the magnitude of that differential. In fact, all the power using devices on the bike don't care about zero volts at all, they care about a difference in potential applied to them at their power connections. In reality the bike would be perfectly happy if the POS terminal was 100V and the Neg terminal was 88V. The bike only sees and uses the 12 volt differential, referenced across the POS and NEG terminals.
OK
So I just checked and both reading were zero.
If you are talking about the tests I recommend, those reading are nearly impossible as it infers the switch contacts wire and connectors have Zero resistance when passing current. Nice dream, but completely unrealistic. All electrical conductors have SOME resistance.
I'll admit it is easier to assume that the Neg terminal is at zero volts. But, that zero volt reference is distributed about the bike over green wires (and frame steel), that have their own connection and wire resistances. When current is flowing, some of the POS voltage offset potential gets applied to any part of the circuit that is separated from the NEG terminal by any resistance.
Easier to show/explain with pictures and diagrams, but there it is.
