Нашёл кое-какую информацию по этому поводу , The change was made for Fermi based cards (4 and 5 series) from 0.5.6 to 0.5.7. GPU-Z 0.5.6 calculated pixel fill rate as "ROPs*core clock" (16*675MHz = 10.8 GPixels/s) while 0.5.7 calculates it as "active SMs*core clock*2" (3*675MHz*2 = 4.05 GPixels/s). The GF116 core, which the 555M is based on has 4 SMs (streaming multiprocessors). The 555M has only 3 of these SMs active (no, you cannot enable the fourth). Сайт правда буржуйский
Different Pixel Fillrate on same notebook models! - techPowerUp! Forums