Also, the adverts are 80% designed for the particular user, based on previous browsing sessions that have been done viewed that computer. Not a space sold by WGT to an individual advertiser.
For instance I was looking for a new anti-virus a while back, for 3 weeks I had bit-defender adverts popping up on WGT after having visited their website.
I think it depends which advertisers employ a particular advertising company, and which advertising company purchases space on WGT. If you have browsed something in line of an advertiser on their list, you might see their product or service on WGT while someone else will see something of a different nature.
If that makes sense.