When a user uses "Album covers" (or a custom view that is similar), has albums with art that is more tall than wide, and in the view configuration "overlay fields inside the artwork" is chosen, the art is stretched to match the 1:1 space MB gives for album art. Due to how the scaling works, the visual defect is only noticeable with art that is noticeably skewed with dimensions (think DVD art vs. CD art).
Albums that are more wide than tall are not affected. They are shown normally with some grey space above the art.
Windows 7, MB 3.3.7367