I see 2 differences in your sample images: size and a horizontal flip.
The size difference could be because you are setting picker.modalPresentationStyle = UIModalPresentationFullScreen;
but then you are displaying it in a smaller popover.
I would try and present the picker without a popover and see if it works.
Also, the preview image looks a bit distorted. If you are using Scale To Fill as contentMode for the imageView then I would change it to Aspect Fill.
I don't know where the horizontal flip might come from.