Most home ovens are not insulated well, and are not all that precise. They maintain temperature by cycling on and off relative to your temperature setting and a thermostat. It is quite common for the actual temperature in home ovens to be upwards of 50 degrees F more or less than the temperature you set on the dial..and that temperature fluctuates as your oven's heating element cycles on and off. So, many people use an oven thermometer to determine the actual temperature inside their oven. Once you know the difference between your actual temperature and the dial setting, you can compensate by raising or lowering the setting. You should also note, that the difference between the setting and the actual temperature might be different in a low oven, as compared to one set at a higher temperature. That is why it is good to keep a thermometer in there, and take a peek at it once in a while. After some time, you will be able to reliably gauge how your oven behaves. For the most part, there is no need for an oven to be all that precise because the kind of cooking we do in there allows for a great deal of temperature variance.