Sgt_Kickaxe - 3:21 pm on Dec 17, 2012 (gmt 0)
It's not rocket science. Google first identifies an ad section on your site and then does not trust any link within that section. If you have links without nofollow in that section they count against you.
Identifying ad sections is relatively easy, especially if it's from a known service, but even if you write the ads yourself and rotate them that *section* of your page repeats on other pages ... and what repeats is of no value.