simple.
the data going through its own cache is not speed limited.
data going through the normal URL route is FUPed or speed limited.
even when there is no FUP but the speed is restricted according to plan you choose, the cache speed is always higher.
just to illustrate this is the speeds friend got on his wi fi for a 512kbps connection
I get ~ 4Mbps from Google servers, even on Google Play Music downloads, which means that it is not caching. This is on a 1Mbps line. So, I get about 5 Mbps bandwidth, max. As for torrent caching, dont care. Planning to switch to 5Net soon, with FTTH.