Dead Reckoning Vs GPS tracks

Hello,

I am just starting with PX4 and drones. I have a quadcopter and I have changed the parameters of EKF2, to disable GNSS fusion. I am using the Flight Review Web App to review the flight log and I can something that I cannot explain:

why is the reference point (lat, lon, alt) of dead reckoning - taken from vehicle_local_position , different from the first point of sensor_gps?