I guess that under the hood there are modes the camera can operate in - lower-res "video" mode and a high-res "still" mode. When you're looking at the viewfinder it's kind of a video - so it'll be lower res and presumably will skip some corrections involved when a still is taken.

Edit: oops I thought this was in reply to a comment where someone complained that the photo was taken from a screenshot of the viewfinder instead of using the camera functionality. Been a long day :-)