I was pondering the X-Boat route from Tenalphi to Strouden in the Spinward Marches - which, as everyone knows, is 5-parsecs long despite X-Boats being J4 - and settled on an explanation IMTU.
Spinward Marches 2127 doesn't exist.
Jump Space goes non-Cartesian for that parsec. You can't jump to that hex. Jump distances measured across that hex are one parsec shorter than they would appear to be. It's like... oh, I don't know... someone cut out that hex and stretched the map around it in to fill in the gap.
Now... who could do a think like that? And why would they want to erase an entire hex in the Spinward Marches?