This is just a what-if type of guess thinking about the way the brass was made....
Could it be that the small shoulder is of slightly greater thickness,
and,
with the first firing a little stretch happened in the body,
and,
that slightly thicker part became the very deepest part of the cylindrical bore of the neck?
Easy enough to check for if that's the case (oo, bad pun!).
But even if so, how could it cause that much resistance?