A mistake is the most likely outcome honestly. Two different people or even teams worked on the different characters and neither was fully integrated into figuring out smaller details like how many fingers there are and such.
Better yet, the Runecarver was never meant to be the Primus but something changed during development, or even the similarities are actually just coincidences.
If I would guess I would say this could likely be chalked up to insufficient communication between teams due to Covid.