Not sure exactly what would cause that, but:
Do you have an image defined in Edit Explorer parts \ Explorer and IE \ Explorer windows \ Glassy Areas \ Top glass area?
What do you have the OS Caption Height set at? (Titlebars and Window Frames \ Edit per pixel frames \ Borders \ Caption [Extra Settings]tab / OS Caption Height) I've never understood exactly how the "correct" number is determined, but it tends to run from 25-30. Your captions are a little tall, you might need to bump it to to something a little higher, like maybe 30-35.