For cannons and beams, explosive effect is hardcapped at radius 1 although it is capable of increasing it to radius 10 on paper.
This causes it to reach its max radius after hitting 10% effect ratio and the surrounding blocks will always be hit by normal damage/6
Since this is capped anyway, perhaps the damage that is spread gets affected by the effect ratio and you automatically set it to radius 1.
surrounding damage for cannons,beams = normal damage/6 * effect ratio