What is this a header bar for? I know a lot of times, in the footer, for example, the main action button is centered - which is maybe why Add Event. And cancel might be left due to its less thumb-reachable position - it’s hard to accidentally cancel something if you have to reach for it. I’m not quite sure I have seen a header that looked like that, though.
Definitely a question for some mobile UX people, if we have any around, as the above is just my thoughts, and I’m not one.