They made it where certain skills improve each other. For instance, Blizzard gets bonuses for every point in cold bolt, ice lance, and what have you that you have. They pretty much did this to get people to use skills that they usually never used. Like, for bowzons, they would always get multi shot and guided arrow, so now those don't even have synergies, but the whole line of "cold arrow" whatever skills all synergize, so they're better.