Ok I think I've figure out why they chose Ghost fpr Decidueye. Deciduous trees are the ones that can completely lose their leaves in the winter, in Decidueye's case that'd most likely equate to all its feathers.

So perhaps the reason Decidueye is Grass/Ghost is because during the winter it sort of dies in a sense of losing all its feathers and is then born anew in the spring, lile a phoenix in a way. Of course logically Sawbuck should be Grass/Ghost too but it's not so I'm not 100% confident in my argument but it's the best one I can think of.