The MacBook Air Mid-2011 i5 variant uses the Intel 2557m CPU. Its base frequency is 1.7 GHz and can Turbo Boost to 2.7 GHz. The frequency is increased in steps, where 1 step is 133 MHz. The processor may operate in Turbo Boost mode as long as it stays within its thermal (TDP and maximum temperature) limits. It seems as though your MacBook is perfectly fine. I wouldn't worry about it.