Protruding resultant vertex when edges are merged

I’m following the bridge modeling tutorial here

when it comes to “closing the top” in the tutorials words, it says that before I merge the edges of an square, I should extrude the edges and then scale them down slightly.
Just to see what happened, I tried merging without extruding. The resultant vertex in the middle of the square pokes out … it’s not at the same level as the sides of the square.
Why is this?
I’ve added a .blend file so you can see what I’m talking about.
Look at the bottom edge of my model in front-ortho view… why does the bottom vertex stick out like that? It’s would be easy to push it back in… but why does it happen in the first place?


bridge colomn.blend (192 KB)

I have made a short video (requires flash)
The first way shown is the same as merging before you extrude. The second is the correct way so you extrude first so you form the flat end.


ah… thanks. Just seeing it as a video helped me solve the problem. Thanks a lot!!