Wring Out is a weird one; I'd agree that it'd make sense as a physical move, but then again, so would Draining Kiss and that's also a special move.
With Petal Dance, I think it's because it was introduced in Gen 1 where the physical/ special split was based on moves. Come Gen IV, they introduced Power Whip and Wood Hammer, along with Leaf Storm, so I guess they wanted to evenly distribute the powerful Grass moves between Special & Physical.