Grey vs Black Rating Stars (iTunes) [screenshot]

[Yervand]

http://img33.imageshack.us/img33/696...20221at110.png

Here on the screenshot, you could see grey and black rating stars in iTunes. What do they mean? If I click on the grey star rating to give it a new or the same rating the stars become black.

What is it?
What could be causing it?
Anything else I need to know about this?

Thanks,
Yervand