Why SITL route is not a straight line, is like a spline

I use Mission Planner 1.3.71, start rover SITL, and set to guided mode, use fly to here function, but the vehicle direction always change, and the route is not a straight line.

Then I make a waypoint planner, write to the vehicle and set the mode to auto, and the result is the same. Dose some parameters affect the result, and which parameter I should set?

Thanks a lot

Use Latest Mission Planner, preferably BETA.