Traditional press designs result in horizontal pressure on the bottom end of the ram during use, which over time and/or with heavy use, wear out the ram and the bore in the frame through which the ram moves up an down This causes an arc in the up and down motion of the shell holder, rather than straight up and down. The only fix would be to re-machine the ram to restore it to a true cylinder. Then the frame can be bored out and a precision bushing installed. But it is almost always cheaper to buy a new press.
The Forster Co-Ax design avoids most of these misaligning forces in the first place, ensuring better alignment over longer life.
Andy