This happens when the top sizing margin is too large and is mainly a problem with themes that have lots of detail in the frames and large sizing margins need to be set, if you look at the default aero theme you will see the top sizing margin is set to 6 pixels, this will allow he rest of the image to be compressed or stretched and allow the top 6 pixels of the image to remain normal, you can have a larger margin depending on the size of your padded border and sizing border in system metrics, the larger you set the border the more you can increase the top sizing margin without it being effected.