Reputation question

Hy.

Until now I didn't understand why when a users get a reputation some have a green dot in front and some have a gray one. I'm talking about the reputation table in the User Cp.
And why some appear but they don't count to the total.

Can some one explain me that.

Thank you.

Green => User who gave you the rep has +ve rep.

Grey=> User who gave you the rep has no power (possibly their rep is in -ve)