Why do people insist on saying "5.56mm NATO chamber," as if all things that were designed to shoot 5.56mm NATO have the same chamber?

The M249 has a difference chamber, and shoots 5.56mm NATO.
The M27 (HK416) has a different chamber and shoots 5.56mm NATO.
The Stoner 63 had a different chamber to the M16, although it was gone by the time 5.56mm NATO came along.
etc
etc.