To elaborate...
Slower weapons usually hit harder than fast weapons per hit. Before 1.1, special shots such a Arcane, Multi and Aimed shot got the benefit of those bigger hits, so overall DPS would be higher, even if two different weapons had the same DPS with different speeds. Once Blizzard noticed that, they decided to change the special shots to "pretend" that the weapon speed was all the same. That way, an Arcane shot with a 3.2 speed weapon would be exactly the same as an Arcane shot with a 1.8 speed weapon.