This is because the way to evaluate results at intermediate surfaces is slightly different than at the image surface. For features which require a temporary image to be formed (PSF, MTF, diffraction encircle energy...), this new image surface is set to be a standard plane surface. A paraxial marginal ray height solve is placed on the selected surface thickness to place the image surface at paraxial focus for the selected surface. The analysis computation then proceeds at the newly created intermediate image surface, which is not necessary at the same position of your real image surface and, therefore, could show different results.