as the the "theory" is that Electrons move from the - plate to the + plate INSIDE the source or battery... giving the + terminal an abundance of electrons, or MORE than the - terminal, and these little boogers are trying to stabilize or go home from whence they came.
If the battery is left alone for long periods of time... the electrons will eventually go back from whence they came or simply die off.
Give them a chance to flee the + plates and terminal via an external source... like wiring through a switch and then through the element of a light bulb and then more wiring to the chassis and then back to the negative terminal of the battery... they'll take the long way around just to get home!
TA-DAAAAA!
Maybe this is why Chrysler for many years wired their batteries in their cars with + to ground and the - to the fuse box... Thinking electrons flow from - to + OUTSIDE the source.
I guess it really doesn't matter but ya just gotta know what the engineers were thinking!