Why does Google show PSA's so much?

Some of my pages are just showing PSA's now and sometimes there is nothing there. It does not look good because it just leaves a lot of white space in the middle of the text. Yesterday, They were all showing relevant ads but some pages aren't.

It could be because I uploaded my whole site this morning after making some changes offline. Perhaps the Google media bot needs to come around and go through the pages.

Is there a way of showing alternative ads when Google is not displaying any i.e. Yahoo in exactly the same positions as the adsense?

 

By the way, my site does not really have any problem regards to irrelevant ads. I am sure there is a very large inventory of ads for some of the topics.

 

Here's a way to not only show your own PSA's in Google, but also track which pages are generating 'em. I don't know of any way to currently do this with Yahoo.

 

Interesting, I never thought to really track it, but by happenstance, I do have that information... Interesting results.

Some days were really bad, but even the good days (yesterday) showed quite a few of my alternate PSA ads.