Erratic Pace?

Anyone know why I would show such an erratic pace on my VA4S? It's a treadmill walk, arm continually swinging (not stopping or changing pace at all and not hanging onto railings).

my Vivosmart never had erratic pace like this (same activity, the few dips in pace were when I did grab the railing a few times but it's not often)

Is there something I'm doing wrong to cause such an erratic pace on my new VA4S vs my old Vivosmart?