Okay, makes more sense. A lot of factory chambers and many customs are cut with reamers that allow the case to over expand in loose chambers. Many dies have trouble resizing the area directly above the belt. So after a few firings, the case is hard or impossible to chamber. The collet die resizes all the way to the belt.
I run several belted magnums, cut with match chambers and use a Redding body die for resizing. I do not have any problems with this setup. In previous factory belted mags, I used the Willis collet die, when needed.