How accurate is it? Running seems to work fine, but when I walk, it never seems to register my steps, and underestimates from but 20-50%. Is there something wrong with the way I walk? I don't pick my feet off the ground very much. I can't even calibrate it as walking a 1 mile will only register 0.5-0.7 miles with the sensor. Hope this is the right forum to ask this in. Thanks!