Don't discount the possibility that the problem originates from your meter.
Do you recall what the lighting conditions were for your tests, along with the actual exposures used for the EI 25, EI 50, EI 200 and EI 400 exposures?
Do you recall what the lighting conditions were for your tests, along with the actual exposures used for the EI 25, EI 50, EI 200 and EI 400 exposures?