Hi,
It was because of two things:
1. you had the separate group ordering enabled per post type – it means that if there are results from multiple post types, some of them are prioritized over others: http://i.imgur.com/D9zzvRk.png
2. Because you also search and return terms as results, the term group priority was higher than the post type priority: http://i.imgur.com/QjeHnap.png
