I have a number of custom image sizes. I noticed earlier, when I was publishing a new post, that the post image was cropped AND scaled. It is exactly the right dimensions.. but it isn't the portion of the image it should be.
This is the code in functions.php, for this particular image size.
add_image_size( 'post', 800, 350, array( 'left', 'top' ) );
Take a look at the original size:
Now look at the "hard cropped" version:
It is practically the same image, only scaled..