With regard to your point about the sensor capturing a different image to what you saw in the viewfinder, this might be due to the jpg processing. The jpg you get includes a number of corrections, most likely being lens correction. Your lens tells the camera what it is and the camera's firmware has a set of corrections specifically for it. If the camera corrects for barrel distortion, it will make the picture like a pincushion and crop it to the rectangle and visa versa. Take a RAW+Fine image and switch between the two and you will see what I mean.
If it's a question of exposure, the viewfinder is just a viewer, it does not have jpg maths applied to it so things like colour balance and exposure will be different to the final image.
