If I had to guess, I'd wager it's because a varnish applied to a bow doesn't really have any logical effect on the missile being fired. I think that any bow/crossbow's ability to bypass DR represents the fact that it fires the missile so fast that it hits hard enough to punch through that protection. Applying a varnish to arrows on the other hands makes logical sense because that's the actual thing that's hitting your target. That being said, if certain varnishes aren't applying to the ammo it's likely an oversight but all of this is just speculation on my part so take it with a grain of salt.
