Strangeness when AF calibrating

When you try to calibrate your lenses, Oly lets you enter the AF adjustment page, dial in an offset value (say -20), then you can take a shot, change the offset (say -18), take another shot, change to -16, etc etc, and keep doing a whole bunch of shots without needing to exit that particular menu screen, or even permanently register an offset value. So you can take a whole series of test images very quickly.

You can then exit the menu page, review the images (and use the lightbox mode if needed) and see which images are the sharpest, and it SHOULD show you the AF offset value on the rear screen along with other shooting data.

What I'm finding though is that although I'm stepping through the sequence from -20 to +20 in steps of 2 say, doing nothing different between shots apart from pressing the rear keypad to increment the value, only SOME of the captured images show the AF adjust parameter dialled in. Some images have a blank in the area where I'd expect a value to show.

Has anyone else noticed this? I've tried to do a search here but I couldn't find anything mentioning this strange behaviour.