I decided when I made the original 6.2.1 that I didn't need isolinux, and could gain a few bites
for the persistent image by deleting isolinux before allocating the maximum space to the .img.
When I went to apply Klaus2008's idea, I'd forgotten about this edit, and as a consequence
was a few bites shy of enough room for the original's .img.

aka shooting yourself in the foot.