Looking at the screenshot, it looks like you have used different positions. Have you used any dynamic positions with Item ID? It looks like only when using dynamic positions with Item ID, the position doesn't show up in the stats.
With Ads Manager you can also use dynamic positions with Item ID position criteria option to achieve the following results:
1. Display ads after the first post if there are no unread posts, else display after the first unread post only.
Position: Below thread post x container
Position Criteria >
Item ID: 1, U
2. Display ads after a random post if there are no unread posts, else display after the first unread post only.
Position: Below thread post x container
Position Criteria >
Item ID: R, U
3. Display ads after the last post if there are no unread posts, else display after the first unread post only.
Position: Below thread post x container
Position Criteria >
Item ID: L, U
As you can see, there are many configurations possible. The above are only a few examples.