Distance Discrimination Challenges

I only have an A Frame in the yard and use it for distance discrimination as part of a larger course all the time. Wyatt and I have gotten good when the tunnel is on the outside of the A-Frame. I can cue him to the outer tunnel by cueing early and often. I can also cue him to the inner A Frame by using an RFP from a distance and again cueing early. When the tunnel is on the inside, it is much harder to get the outside A-Frame from any kind of distance. Kind of a puzzle to me as to why this is so much harder.