Slitherine

How do you know which army you will be playing when accepting an Open Challenge ?

I have accepted and thought that I would be playing the first named army only to find that I am playing the second, which is not what I wanted.

On other occasions I have found the opposite, I play the first named army.

I think that in all the challenges I've played, open or otherwise, the person who sets it up gets the first named army, the person who accepts gets the second.