It's possible for high tension voltage to flow though the secondary loop with the firing of only one of the spark plugs.
Here are two examples:
Old hard cracked worn-out spark plug wires may leak voltage through the insulation and short to or partially short to the engine head.
Fooling with the less than perfect plug wires may randomly leave them in a position where there's more or less leakage depending on their proximity to the engine head, which can produce mixed and widely fluctuating results during testing and riding.
The high tension leakage can get so bad that the rider can feel a buzz when touching metal parts on the bike, such as for example upon switching the petcock to reserve.
Spark plug covers/caps/boots often have a built-in resistor.
Dyna solid core plug wires come with non-resistor caps already built in.
Good Fortune!