Because it sounds coool?
Apparently William Green made a mistake in several of his books back around 1960 and most books that base their numbers/information on his books repeat the mistake.
Admitting that the K-4 used a pair of MG 131s and MK 108 would mean the 109 had not increased it's armament in several years (when ever the MK 108 was first used) and was getting rather under gunned compared to the newest allied fighters.
An honest mistake, wishful thinking, bias, your choice of mix?