Flat damage reduction (e.g. -2 damage from every attack) makes a single large hit better than multiple smaller hits of equivalent total damage, since the damage reduction would only apply once with the single large hit. So one hit of 40 damage would be better than four hits of 10 damage each.

40 - 2 = 38

(10 - 2) + (10 - 2) + (10 - 2) + (10 - 2) = 8 + 8 + 8 + 8 = 32

Proportional damage reduction (e.g. /2 damage from every attack) means that, rounding issues nonwithstanding, there's no difference. It doesn't matter how many smaller chunks it's divided into since the amount you subtract is based on the amount you're dealing.

40/2 = 20

(10/2) + (10/2) + (10/2) + (10/2) = 5 + 5 + 5 + 5 = 20