Why not 50 mm when i tell him so :)

Hey Guys need some wisdom for you all!!! Why is it like that, when telling in Vcarve pro or any other CAD programm a circel of 50mm and tell him to cut in the inside, why its the finisht circle not 50 mm in the inside, but is about 52.76 even if i have put the stepover at 0.1 mm 1.8% ???