If you imagine what it would look like without the merging, the one shown would be the one nearest the top of the screen - so if you’re sorting by ‘New’, then it’ll be the newest, by ‘Top’ and it’ll be the most upvoted, by ‘Active’ then the most active. Because of how much LW dominates, it’ll pretty much always be that one.
Nope - there’s no setting to change this behaviour.