Don't understand why this draw lags

Questions
1

function init( self )
go.animate(".", “position.x”, go.PLAYBACK_LOOP_PINGPONG, 300, go.EASING_LINEAR, 2)
end

function update( self, dt )
local center = go.get_position()
msg.post("@render:", “draw_line”, { start_point = center, end_point = center + vmath.vector3(0, -100, 0), color = vmath.vector4(1, 1, 1, 1) } )
end

I think that the vertical line should be exactly at the center of game object.
But when the gameobject is moving, the line lags a several pixels back.
Why that happens?