The idea is the shortest plug leads go to the bottom plugs.
Right mag to right bottom, left mag to left bottom.
Long leads mean weaker spark. Bottom plugs foul the easiest, so should have the hottest spark available, so, the shortest plug wire.
The threaded portion of the plug is screwed into the head, which is ALWAYS negative. The engine case is grounded to the negative side of the battery.
The lead is positive and so it arcs/sparks. If the polarity to the plug is reversed to also being negative, then how could it fire. You can't make the case positive so this theory is going nowhere.
Best,
Timing has nothing to do with it, the magneto spark is positive on one lead, negative on the next all the way around the mag. One polarity causes most the wear, I don't recall which, but moving the plug to a position of opposite polarity will equalize the wear among the plugs, by virtue of the polarity and the lead length. There is a correct pattern to to rotate them, I'm not at all sure it is simply top to bottom.
Tim