As baxslash says, you are comparing the image height to the sprite height, which can be two different values. If you want your sprite to match your image dimensions, I recommend setting the virtual resolution at the start. Then as long as you are using the image to create the sprite or are sizing it to match, it should match.
If you are not setting the virtual resolution from default, it is set to 100x100. So if you are resizing the sprites down to match that, the image will never be the same size.
However, looking at your code snippet you already setting a resolution :p
Are you setting it to match in your setup file? If not that may be the culprit.
Lastly, are you running in a window, a maximized window, or full screen? I'm not too sure how AppGameKit handles fullscreen but if it is scaling to your resolution, as it seems to when maximized (obviously), it may also change your image and sprite values accordingly.
Oh as baxslash mentioned also, the screen ration will change things. But I assume you have not changed that in the code beyond your set resolution command. But if you are using a widescreen monitor, which is likely, 640x480 is not the correct display ratio for fullscreen or maximised. So you will need to rethink your resolution settings a little to work for your monitor.
So if there is any relationship it must be your display ratio.
