Images Cropped When Width = Custom Header Image Width

[questions4wordpress]

I am using the twentyten theme, which I have modified. I have changed the header image's size via the Appearance Editor. The image is exactly the width of my website, 900px. When I upload an image whose size is also exactly 900px, the images height is cropped to the height of the header image. If I upload an image that is 901px or 899px, however, the image height is not cropped.

[nacin]

Can you provide complete dimensions of some example images? I don't quite understand your explanation.

[solarissmoke]

#18799 has a better explanation of what the problem is, and where it is coming from.