iPhone Xr Axure Dimensions

Hi,

I set up a mobile prototype with iPhone XR dimensions (414x 896)
However, when viewing the proto in my iPhone XR to usability test, the aspect ratio isn’t centred - the only way of getting a centred view is to have a base canvas of (378 x 896).

Hoping someone can help me to ensure what I work on in the Axure 9 Page dimensions show accurately in the view.