Google Global Cache (GGC). GGC is a cluster of Google provided servers installed inside an operator’s network to improve performance by caching popular content locally. Serving content from the edge of an operator’s network eases backbone congestion and relieves traffic on peering and transit links, saving cost and improving QoE.
Some of us live in the wrong cities and watch videos that have not been propagated to servers across the network. Youtube likely replicates popular streams across more servers – whether those servers are close to you “depends”. Anyone else in your neck of the woods watching that video? If yes, it might have been propagated to your area, but if not, it sucks to be you.
This explains why the top viewed, top rated videos all stream great, but the obscure videos I sometimes watch (like sample video output of obscure cameras) never get cached locally to me.
Or so the theory goes!