Thread moves so fast so quoting myself if someone knows why does it do this.
I don't want to spend all my points in Skill just to realize it doesn't really help with the damage.
The MLGS is split damage, so this means the scaling with arcane while an A is going to be less than Strength or Skill. If it increased at the same rate as str/skl when you leveled up Arcane the damage for the weapon would be completely unbalanced. Thats how from decided to balance split damage weapons, same way with the BoM and Burial Blade.
Now the reason your getting more damage increase by leveling skl over str is because your strength stat is most likely at or above the soft cap, which is 25. After that the increases arent as large. Leveling up Skill till you reach the soft cap will give more of a damage increase because of the B scaling.
EDIT: Just noticed you said you havent reached the soft caps yet. Interesting...perhaps the scaling letter is lying and Skill has an A while strength has a B? How upgraded if you weapon?