cyclometer - why does it ask for tire width?

[pccp]

I just got a wireless cateye cyclometer and set up the magnet about 3/4 up the spoke. The sensor is working but when setting up the cyclometer it asks for a code for tire circumference and width. The code book has different codes for different width 700c tires. For example theres one code for 700x25 and one for 700x28 (my size) that are different.. Why does tire width matter? I don't understand, shouldn't width make no difference?
Also, why does where you put the magnet/sensor not matter? Basic science that I'm not understanding :(

[landshark1]

As the width of the tire increases, so does the height, which changes the tire circumference.

[Skyler_WA]

Also, why does where you put the magnet/sensor not matter? Basic science that I'm not understanding :(

The computer is measuring the amount of time it takes for each wheel revolution. No matter where on the wheel you place the sensor, it's going to measure the same amount of time per revolution as all the other possible sensor locations.