Not sure if i know how to explain this but here goes! Say i was to cast multishot about 10 yards away from a mob, what happens if there are mobs further away? will they receive the '10 yard damage' or will they receive more?
The short version is that Zei's Stone of Vengeance calculates it's damage bonus per monster hit. So, if a player casts Multishot just once, then monsters farther away take would more damage than those that are closer.