Edit: Judging by your edit, you seem to be confusing "what should they show" with the original question, which is something like "Why is the right 50% plus the top block of the left column ads?"
The answer to your question is "whatever links match the search terms based on their wonderful, wonderful indexing algorithm."
The difference is that one is what Google think will be the most useful results for users, and the other is what Google think will make them the most money regardless of usefulness.
http://i.imgur.com/IhQLUoD.png
Edit: Judging by your edit, you seem to be confusing "what should they show" with the original question, which is something like "Why is the right 50% plus the top block of the left column ads?"
The answer to your question is "whatever links match the search terms based on their wonderful, wonderful indexing algorithm."