AN standard naming convention when they started putting it on everything, same reason why an M2 is an M2. Navy specific things get there Mk designation, but there is a push to move away from it for joint service weapons. Of course, that isn't always the case...the Mk46 and Mk48 are both torpedoes (though, the 46 now replaced by the mk50) and machine guns. Basically, the navy is weird.
Given that, it's entirely possible a M134 is also a Mk44, in keeping with the "is it man portable or not" confusion of navy nomenclature weirdness between NAVSPECWAR and NAVSURWAR.
Heh.
M1 Tank, M1 Helmet, M1 Submachinegun, M1 Rifle, M1 Carbine, M1 Bayonet, M1 Mine, M1 Armored Car, M1 Combat Car, M1 Mortar. Welcome to the Army/Navy nomenclature system.
Minigun = GAU-17, AN/M134, Mk44, Mk25, GAU-2..... Were they giving away promotions for renaming stuff or something?