It's hard for me to get at this. But I think what you are saying is that it may simply appear that you are not stable, when in reality you are stable. Is that correct?
Appearance really doesn't matter when we get down to the practicality of what we are doing. As long as you are stable, I would say you are stable, even if it looks/seems like you shouldn't be.
Now if you are physically stable, we can ask, "what is making you stable?" Is it your body, or is it some other "force"? If it is your body (your structure) making you stable (aligning), it is a body skill we are talking about. If it is some other "force", then we can take your body out of the equation, and we'll need a new definition, one that doesn't include "Body skill".
So appearance aside, is the body making itself physically stable, or is some other force making to body stable?
I think structure and alignment is the normal way to prevent your body from buckling and toppling. I think structure and alignment is expressed in the physical orientation of body parts, like stance and bracing. I think most people can recognize it for what it is.
With internal training you condition your body. Over time it changes your tissues. It unifies your body. This allows you to use your body in ways that would not work with a normal body, because the physical orientation of the body parts would preclude stability from structure and alignment. In those circumstances a normal body would buckle or topple.
So I think it is the body itself (lead by the mind) making itself physically stable, but structure and alignment is not all there is. A conditioned body can derive a lot of physically stability from its own integrity and organisation, independent from stance and bracing.