It's normal. If the voltage is too low, the hardware will compensate (by either doing error correction, re-sending signals, etc. - unless it's too much to compensate, at which point there will be a crash). It's the same with CPUs, too (albeit it may not be as noticeable as they tend to crash the system quicker). That's why you have to tweak things, you need to find the voltage/clock sweetspot for your unit where it performs optimally. It takes time and patience but in the end it's worth it. Also note that this sweetspot can change slightly over time.