I've noticed that even SteemVoter too skips voting some of the posts, sometimes it shows them voted up but in reality it hasn't cast any vote there.
I'm planning to move on to SteemAuto now though it's in beta as of now but would like to give it a try. How about you?
I only noticed that it does this when you already voted manually, before steemvoter kicks in. Then it will show an X.