Interesting — the canvas is set to Screen Space - Camera. And the Canvas Scaler is set to Scale With Screen Size with a reference resolution of 3840x2160.
I never quite figured out how canvas scaling works, so I can see why it's a little confusing. To clarify my original post, those screenshots were taken in the editor with the Game view set to 1080p. I've noticed similar behavior in builds, but I figured I'd point that out, because I think some screen-related properties return different values in the editor (like Screen.width or one of those).