Probably just for the sake of consistency, which is good. The bigger kustom weapons, the Kustom-Mega Kannon and the Kustom-Mega Zappa have the same wording, but multiple shots.
Essentially you can only get one mortal wound from each weapon, but multiple weapons can zog you multiple times.
So, if you roll 4x
for your dread, it takes take 4
MW.
If you roll 4x
for your mek gun with
KMK, it takes 1
MW.
If you roll 3x
for your Morkanaut's zappa and another
for its KMB, it takes 2
MW.