spark location the less timing needed. When the spark plug is in the center of the chamber, the flame front only has to travel half the distance in all directions to completely burn. In a wedge combustion chamber, the spark plug being on one side requires more timing advance cause the flame front has more distance to travel. That is why chamber design and spark location can determine how adverse the combustion is to detonation and how much boost you can run safely one pump gas.
When you move the spark plug closer to the top and you keep your spark timing the same as longer spark plug, you are essentially retarding flame front timing. Making it less prone to detonation. That is my theory how I see it.