Yeah, lechat is right. I didn't even realize the full meaning (lack of understanding) of your question. The sensor doesn't count distance, it counts revolutions. Because every modern cyclocomputer has a place to input the circumference of your wheel, all it needs to do is count the number of revolutions and multiply by the known circumference to get the total distance traveled.
Since every part of the wheel undergoes the same number of revolutions the sensor can theoretically go anyhwere, even though the radial distance traveled by the sensor will vary depending how far out from the hub it is. It will still only cross the magnetic pickup once per revolution, giving the same set distance every time.

The circumference you program into the cyclocomputer is sometimes measured in inches, sometimes centimeters and sometimes millimeters. So even though your computer is displaying your distance in miles, behind the scenes it's actually counting total millimeters (or cms or ins) traveled and doing the appropriate math conversions.

The best thing to do to have your computer give you accurate readings is to make sure you get the circumference programmed in correctly. There are general numbers for different kinds of wheels/tires but the best way is to measure the actual wheel on your bike. For instance, put a mark on the ground and line it up with your valve stem. Roll it forward one entire revolution until the valve stem is again in the exact same position and mark it. Measure the distance between the marks. That's the number that goes into the computer. For even more accuracy, sit on the bicycle while you roll it so it takes into account your weight pressing down on the tire (which will change the effective circumference slightly).