Good spotting! This is a known issue, we haven't looked at fixing it yet because it's just a minor visual thing.
It occurs because when you edit an existing line, it breaks up the line into two separate lines - in this case one from the circle station to the cursor, and one from the wedge station to the cursor. Because now you have two separate line both crossing the water, they each use up a tunnel. If you drag the cursor to a position where only one of the lines is going across water then you'll see the tunnel count tick back up to two.