Google turning away advertisers due to lack of supply?

What are they lacking in terms of publishers?

[HughMungus]

Reading an article about ads in RSS feeds which mentions "This October, Google has had to turn away advertisers due to lack of supply." How is this happening? Does this mean that Google lacking certain types of sites on which to advertise? If so, which types? If not, what does that mean? Is this a common problem with Adsense-style ad networks?

[hunderdown]

If that's true, that's good news for the publishers in the areas where there isn't enough supply. I would assume this is only particular areas... And that increased competition might drive up bidding.

[shafaki]

Makes me wonder why on earth have they refused my participation in their RSS/Atom ad program twice for two different blogs though I used their regular AdSense for over a year by now.