Others here have made reference to the Eyewall Replacement Cycle. This is a natural evolution of the tropical cyclone as it cycles through periods of strengthening and weakening. The radar iamges we see show a "slice" of the atmosphere just above the surface of the earth. What is going on throughout the entire volume of the atmosphere is quite complex, though. What we see on radar is usually a response to other factors in the atmosphere - like dry air intrustion, and the weakening of an eye wall as winds weaken and spread out.

As we saw with Andrew, hurricanes can rapidly strengthen as well and produce winds at the surface even higher than what might be estimated from radar. Lots of research from people much smarter than myself continues to be done on how to best analyze and forecast these short-term changes.