Picture taking and trimming issue

I’m currently trying to implement a screenshot feature inside my application, my problem is I want this picture to be crop to a certain area that the user see on the screen (like a preview) but when I take the capture and crop it, the final picture doesn’t fit the area that was shown previously.

I think there is maybe a problem with the screen itself because it seem’s to not be totally horizontal but that’s just an idea. And I’m also wondering which camera is currently taiking the picture …

If anyone have other ideas about that or other suggestions, thanks in advance