It seems to have something to do with automatic thresholding. If I cut out most of the black background it seems to work just fine, if too much of that black is included no characters are found. This presumably means that somewhere there's a thresholding that if theres too much proper black (or white if you take the negative of this image) the entire gray area is lumped together into a unseparated white region?
(Our vision is very good at this task, computers can still be easily fooled by things we would never fall for. On the other hand: optical illusions...)
HTH
