The rating scale analysis groups all students with the same score and all items with the same difficulty. The end result is a set of nearly parallel lines connecting the starting and ending convergence location of a student or an item. (Closely spaced locations have been omitted for clarity.)
This increased the student ability mean and decreased the item difficulty mean. Culling increased the spread of both distributions toward higher values.
The item difficulty mean migrated to the zero logit, 50% normal, location in all four analyses: full rating scale, culled rating scale, full partial credit, and culled partial credit. Winsteps performed as advertised for psychometricians .