Sensor readings are rareely 100% accurate or precise. This is soemtimes due to limitations in the sensors themselves, soemtimes due to the environment (dirty, poor visibikity,etc.), and soemtiems due to the processing that occurs between the raw sensor readings and values returned (for example if there is a low-level AI algorithm). Higher level processing often needs to use some form of reasoning with uncertainty to take account of this.
Used on Chap. 19: page 469