Clean ovens function better
If your oven has become very dirty and layered with carbon you may notice that the cooking times increase. This is because a dirty oven does not function as well as a clean oven.
The elements inside an oven heat up and then that heat is rebounded around the oven off the enamel keeping it hot. If however your oven is very dirty then that heat does not rebound around the oven as well and you are just cooking the carbon that is coated onto your ovens enamel surfaces before you are cooking your main food.
This is obviously not good practice as you want your oven to cook the food you placed into it first and not the carbon deposits. You should notice that after your oven has been professionally cleaned your cooking times reduce slightly.