From the ATC 2,000 Miler Application:
"The Appalachian Trail Conservancy (ATC) recognizes anyone who reports completion of the entire Trail as a “2,000-miler.” The term is a matter of tradition and convenience, based upon the original estimated length of the Trail. Conservancy policy is to operate on an honor system, assuming that those who apply for 2000-miler status have hiked all of the A.T. between Katahdin and Springer Mountain, either as a thru-hiker or in sections."
My certificate says "In recognition for having hiked the entire Appalachian Trail"
I guess some people think you only need to hike 2,000 miles but the application is clear that you need to hike the entire trail, with some exceptions for emergencies.