robot don't spin correctly

hi every one i've make p2dProxy.SetSpeed(0, 2*3.1415926535897) in my code
but the robot don't  spin in a complete circle 
this is the definition of my robot

	size [1 1 1]
	drive "diff"
	bitmap "circle.png"
	robot_laser()
	laser_return 1                 
	localization "gps"
localization_origin [0 0 0 0]
Thank you 



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