Bradleys exhibit a fair amount of temp set point over run and under run. A range of 20-30 degrees is normal. This behavior is common to many electric resistance heaters, such as your home oven.
For most of what you smoke, these temp swings are not a problem. There are some products (fish, cheese, and sausage) that benefit from the level of temp control you can get with a device like a PID, but most Bradley owners find the stock controls are just fine for their needs.