I've occasionally had my bike computer give erratic readings while riding. If I know the sensors are lined up correctly, I've found that the next most common problem is the connection between the computer and the bracket that mounts to my handlebars. If the connection is flaky, it won't always count all the wheel revolutions. In that case, it helps to remove the computer from the bracket, wipe the contacts to make sure they are clean and put the computer back in the bracket, making sure it is fully seated. Just another thing you can check.