Lol thats funny! Its also weird :s. They are so big why would they need to advertise with adwords? - Prilep
It doesn't have to be YouTube. As you know, advertisers can select what url to show and what url the visitor will visit. It doesn't have to be YouTube just because it says youtube.com.
Advertisers have to use the same base domain now, they can't display www.site.com but send people to www.example.com. This was changed a couple of years ago, but there were plenty of advertisers who took advantage of this when you could
I think it is bug in adsense that sometime i see ad of a website on same website. I have reported it to adsense team 2 times but they have not yet worked on this bug. Now i put my url in adsense filter.
It's actually not a bug, there are some advertisers that want this to happen, especially those enormous sites (think something like iVillage with so many related and unrelated sections on it). Both advertisers and publishers can block their ads from appearing on their site tho.