Yes, the position is represented by a number and a member of the dev team tells me in our case that is 6 … what’s probably happening here is that our plugin is registering the menu item first then the theme is overriding that.
Our position is defined in the-events-calendar/lib/the-events-calendar.class.php on line 50 (or thereabouts – look for the term menu_position to zero in on this).
We don’t really recommend editing core plugin code – any changes will be wiped when next you update – but so long as you are aware of that you should be good (or you could modify the theme code).