468x15 link units don't get messed with as often and when the landing page does get changed in a way visitors don't like at least the backpage button brings that visitor back to you instead of off your site.
google is constantly experimenting with new ways to display ads. Usually they run such tests for a couple of publishers for a limited amount of time and if the eCPM is constantly higher they roll it out to everyone.
Seeing this 160x600, four ad links at the bottom where fifth ad should be. Doesn't look too bad, now instead of five ads in a 160x600 there are 8. Not sure if they open in new window did not want to click on them.