There are 2 components to every post (actually there are more, but only 2 we are concerned with). One is the member piece, on the left, where the avatar is. The other is the actual written post itself.
Here is a picture of the member piece.
The part which I have circled is the total overall post count. Every time you post, it increments up one. This has always been the case and has never changed.
The post piece is pictured here.
The part which I have circled is the post number. Posts actually have numbers so that they can be identified by the database. They also increment up one, but in order of posting, and for everyone. Hence Setanta can have post #35, I can have post #36 and Angelique can have post #37. This has always been the case and has never changed.
The next time Angelique posts, her post count (the count of her total number of posts) will go up one.