I don't know why this happens, even though I am pretty good with computers I can't answer this. But here is what I have observed and have noticed this on other websites as well.
After a few refreshes, it defaults to one (probably the most prominent) advertisement which appears to be Mastercard. This is probably why we are having a hard time seeing it. If you right click the main page, you can "view source" and find the code that would link to the advertisement, go800. I don't have the time to do this, maybe someone can find it.