I really don't know, but western Oregon, at least the southern part, is quite a bit more open and drier than western Washington. From what I've seen of southwestern Oregon, which admittedly is almost all just from the I-5 corridor, but there are a lot of oak trees and it reminds me a little of the Klickitat County area here in Washington. So that would be my guess, the habitat is just different and Oregon is more suitable for the other subspecies.