whidden- Read about Fl sizing. If shell holder contacts the Fi die, shoulder may be pushed back to far.
When the 2 dont make contact, a press with sloppy linkage , may produce a variation in shoulder bump.
This is where Redding competition shell holder sets help, with Redding dies.
I set up all my dies for a bolt.gun for a 0.0015 shoulder bump and I use a co-ax press. Coax press doesn't have a typical Shell holder