Grey vs Black Rating Stars (in iTunes) [screenshot]

http://img33.imageshack.us/img33/6969/screenshot20120221at110.png

MacBook Pro (early 2011)
Mac OS X 10.7.3
iTunes 10.5.3

Hello!

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

So my questions are:

1. What do they mean?
2. How are they different?
3. What could be causing it?
4. Anything else I need to know about this?

Thanks,
Yervand

The hollow rating stars are if you rate the album or songs but not the other. Solid rating stars are things that you have rated. I can show you a picture when I get home to better show what I mean.