About CalibrateCamera() Why are the results different every time?

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I try run the sample of opencv3.4.1 smaples camera_cliberation ,I use different kind of Calibrate_Pattern, CHESSBOARD CIRCLES_GRID ASYMMETRIC_CIRCLES_GRID ,same camera fixed and focus fixed ,but ererytime I get different cameraMatrix, distCoeffs. even I use same Calibrate_Pattern .Why?