Fluctuation to a high point then down to a low point is normal for an analog based system. For it to be more precise, you would need a digital controller that would cut off power to the element within a degree or two of the set point and turn it back on when the temp dropped a degree or two below the set point. And you would pay good money for a truly reliable digital control. I would not worry about it. Auberins makes such a control that regulates the temperature digitally if you are interested. But it is not inexpensive. And I do not know how reliable it is. For what it is worth, I would not worry about the fluctuation.
