Thank you all for your replies:
Love Zhaoying: Yes, I agree, there is a higher priority animation running as I comment on below.
Zalificent Corvinus: Yes, correct, just what I think is happening.
Emma Krokus: I have tried it in different sims and get the same results. As you suggest, I will most likely strip all the scripts and animations out of the objects I created and move them to some piece of old furniture. It is not my avatar, I have had 4 different avatars try and the results are consistent.
Chic Aeon: You are correct on the first animation used when sitting. I intentionally made that a simple standing animation for both sitters and for both it shows them sitting when they should be standing. With respect to the viewer, I have not update Firestorm and everything else works fine.
What I did do was “build” a cube. When I sit on the cube, my legs assume a sitting position. This sitting position, at least the hips legs and knees, is exactly the sitting position I am getting with my AVSitter build. That said, there must be a default sitting animation in any cube you build. It is not listed and I do not know how to turn it off. From a curiosity point of view, I would like to know how it works. From a practical point of view, I do not want to spend time fighting with it. I will as I said above just move the scripts and animations to some old furniture and try again.