Exploring the software behind Facebook, the world’s largest site

If I remember correctly, Facebook uses the CDN as a cache for images, and loads from central servers when there's a cache miss. Even with a very large cache, there will be compulsory misses as new photos are loaded into cache. That might help explain their figures.

The CDN has to load them from somewhere.

Mmm, You're right, then I dont understand what photos they're referring to.