I think there could be issue.
Sometimes I have noticed the icon to be green, but still there can be updates.
I just assumed that this is because new updates become available between me starting the computer and when I clicked the icon. Because I don't really know how the software checks the updates.
Asus P7P55D, i5 750 2.6Ghz, 4GB DDR3, radeon 5770, 80GB Intel SSD, Dell 1600x1200, Dual boot, Linux Mint 14 Cinnamon, Windows 7