Howcome in halo 2 the Grunts are against the Brutes,but in halo 3 they are back on the same side as the Brutes?

near the end of halo 2, the brutes betray the elites. you play a few levels as the arbiter killing the brutes with fellow elites. but the grunts and hunters are also helping you! so that means the grunts and hunters were also betrayed by the brutes! so at the end of halo 2, the grunts and the hunters and the elites are against the covenant. in halo 3, the elites are still agianst the covenant... but the hunters and the grunts are back in the covenant! why is this?