What are you attaching (move/merge) to the destination post? A reply-post (from the post edit screen) or a topic-post (from the thread Tools menu)
If you are merging two topic-posts (each having their own OP), the topic-post set that you are merging into will retain its post count. The posts that you are attaching to a new topic will no longer have their view used, since they are no longer the OP post set of the thread.
As a reminder, the display of posts is in chronological order, so if you've attached an older post to a newer post, the display ordering will be oldest first.
