One thing I have noticed is you get a nice little blue arrow pointing to the top AdWords ad (as long as there are ads above the organics) after you have entered your query and hit enter or search.
This draws extra attention to the ad & if they can increase the CTR of the No 1 ad by a few percent they are going to make a killing in extra click income.
It also takes you to that advertisers page if you click enter after your initial search.
If you're ad is is no 1 all the time, have you noticed an increased CTR recently? This could explain it.