Because there is a 2.75" magnum 12 bore shell .
I remember I'd seen them in the past but I found this to back up my claim.
"2 3/4 inch Magnum shells are also popular for waterfowl hunting. These are loaded with steel shot in sizes T, BBB, BB, 1, 2, 3, and 4. The MV of steel magnum shells is usually 1275 fps."
http://www.chuckhawks.com/12gauge.htm
DarkMM
20 years of shotgun shooting