Actually the cycle can vary from 6 - 13 years, and in terns of a full magnetic field reversal cycle, the full cycle is actually twice the 11 year estimate. As to why the sunspots follow this, the speculation is that they are just the surface tracers of the so-called 'solar dynamo' cycle which regulates the entire magnetic field of the Sun. For reasons that we are just beginning to understand, magnetic dynamos such as the Sun's and the Earth's, have periodic or possibly quasi-periodic phases of polarity change, perhaps followed by long periods where no cycles occur. Sunspots are regions where sub-surface magnetic fields are strong enough to pop up through the surface and billow-out into stable magnetic storms. these last from days to months depending on what is going on down below.