Because you are actually checking if their are calibrated. You use these volume sto take measurments and calculate some statistical measures ( standart erros and CV %) and you check if these values are within the values given by the company. The actual caibration is being done by the comalny you bought the pippetes. At least for gilsons. Unless you ve been provided with the appropriate instaments( an allen like key to rotate the base of the pippete so the volume can be adjusted and then recalculate with new measurments to see if you are in the range of given statistical values)

Hope it makes sense

if that's the case, you probably need to send the pipette to the manufacturer to recalibrate as that exceeded what self-calibration usually does: https://lab.plygenind.com/how-to-calibrate-pipettes