When a supported client device (OS X 10.8.2+, iOS 7+) checks for updates, it first checks with Apple as it would without an available Caching server. This makes sense as Caching does not replicate a full catalog of updates from Apple. When an update is requested, the device requests it directly from the Caching server, not knowing if the Caching server has a local copy or not. If Caching doesn't have a local copy it starts to download the update from Apple, and almost immediately starts sending what it has downloaded thus far back to the device. Otherwise, the local copy is returned to the device.
This is one of those save to Evernote type articles if you are a Mac or iOS admininstrator.