Treadmill - 1st mile ok, then slows right down

My VA4S tracks the first mile of my treadmill activity bang on, but after that, the distance counter slows right down. My steps are accurate (as well as custom stride length set in connect), but I end up finishing a 3 mile walk (52 mins at 17.5 m/mi) with my watch telling me I only walked 1.5 miles. Why would the first mile be accurate during the first 17 minutes of my activity then the distance counter slow right down? I have GPS off (using the treadmill activity).