ShopTalk: swingweight question

[DADUGALLY]

Hi Jent's,
I came across something I don't understand.
When mocking up a set of irons and determining how much weight I was going to need to add to get my desired swing weight I would get one # say d-1 before butt cutting, this would leave a few inches of butt protruding past the seat or stop which should if anything give me a SW # that is slightly less than the # I would get after the  final butt trim due to several inches of shaft hanging past the edge at the butt end or so you would think.
Low and behold after butt trimming and rechecking I found that the SW actually went down a couple of points.
How could this be?
David