ummm...maybe something to do with amount of explosive in a round ?
I've got the stats for the M-Goss shells used for those guns as well; a MG-151/20s M-Goss Thin-walled HE shell is 115 grams, compared to 92 grams for the normal round; that gives the MG-151/20 with M-Goss a 1.34 kg/sec burst.