If the diferent dish pistons are all supposed to be the same compression ratio, then i assume that the original combustion chambers for these particular pistons must have been of diferent volumes. It's possible that your shallow dish pistons went under a combustion chamber with a larger volume.

The factory listed pistons in three standard oversizes, .5mm, 1mm, & 1.5mm. The cylinder block should cope with up to 3mm of bore size increase without difficulty.

Have you tried ordering "genuine" pistons from your Datsun /Nissan dealer?