Seating a bullet in a clean case eliminates any variable that caseneck interiors with carbon baked on creates.

Editor: It is a common practice to ultrasonically clean prior to annealing. However, we do recommend lubricating the necks with a dry lube after the cases have been ultrasonically cleaned. Once some carbon gets in the necks again, the dry lubrication may no longer be necessary. However, some successful shooters continue to use the dry neck lube. YMMV.