It could be a spectral response thing. It might be a calibration issue (you know you can tweak the Digisix). I think meters and cameras should agree.

I tested in open shade and in dark darkness of a garage and found my Spotmatic F and Sekonic Twin-Mate (reflective mode) could be made to agree.

I set the Shutter and f/stop from the Pentax onto the dial of the handheld meter. Then aimed and pressed the meter button several times holding it at different distances and angles until the needle matched the match-needle and every time afterwards. So it can vary because of the way you use it too.