AdSense Running Low or Slow?

Something I've been noticing the last few days is my AdSense ad units seem to either be low on inventory or slow as they are inserting a lot more PSAs than usual.

When I noticed this I reloaded the page a few times to see what was going on and it's hard for me to fathom that reloading a page with 3 ad units suddenly has 2, then 3, then 1, then 2, then none, then 3 and so on and so forth.

Would either indicate there is a lack of ads available or perhaps AdSense just can't connect to the ad server fast enough and they drop a PSA when it panics during a time delay.

Brings up many questions:

• Could AdWords be running low on inventory?
• Are advertisers just broke after Xmas?
• Perhaps a new experiment in showing less ads for more revenue?
• Maybe the network is just overloaded?
• Could YPN be stealing that many advertisers?

Whatever the reason, it doesn't seem to be impacting my revenues so far.