PnP method error in estimation ditance


I am dealing with estimation distance of object with PnP method. I already did some measuremnts and my calculated distance is far above actual distance. I also used a calibrated camera. For distance calculation, I use result distance in 3 axes from PnP and distance pattern in 3D:

        tvec = cv2.solvePnP(
                objectPoints = model,
                imagePoints = key_points,
                cameraMatrix = camera_matrix, 
                distCoeffs = dist_coeffs,
pnp_distance = mt.sqrt(tvec[0]**2 + tvec[1]**2 + tvec[2]**2)

Here you can see that calculated measurement (dots) are above real values (line). Is there some extra options to make PnP method more precise, or maybe I did something wrong? Thanks for help in advance!