Two things can affect the readings.

One, if you put the magnet out towards the rim, at high speeds the magnet can be passing the sensor so fast that it doesn't register. It's better to put it closer to the hub.

Two, if you mount the magnet incorrectly, sometimes (probably at lower speeds) the sensor might register each revolution twice, because it will pick up both the north and the south pole of the magnet as a pulse. Most magnets I have experience with wouldn't really allow this but some might.