Both are correct, that's why it says high performance settings. It means it's 955 when you're running games and lower when you're not. (This is meant to save energy.) GPU-Z always shows high performance clocks to avoid confusion, CCC shows actual, current clock speeds.
To get an actual reading with GPU-Z, use the sensors tab. You should be able to see the current clock speed there.
CPU: AMD Phenom II X4 810 @ 3120MHz | RAM: Kingmax 2x2GB DDR2 800 @ 833MHz| MoBo: MSI K9A2 CF v1.0 (BIOS: 1.D)| GPU: Asus HD 6850 1024MB (DirectCu) @ 835/1135MHz | Display: L24FHD | PSU: PC Power & Cooling Silencer 750 Quad | OS: MS Windows 3.11 Pro x64