The portraits move around a bit on both the single colors and the bicolors, but it is true that most are nicely centered. I am guessing that De La Rue's quality control is responsible for the general high standards of the printing.
I just made a quick pass through a hundred-plus stamps up to the 1/ value, and I didn't see any that had the portrait touching the frame, but a good number were shifted off of center. Though I don't have a cite for this, that tells me that they had to be printed in two passes.
The printer had to be awake for this one...

Too bad the perforator was out late the night before.
(Scott 4a, SG 5a block)