Cache survived varnish restart?

[CJ Ess]

I saw something today that I can't explain - I upgraded Varnish 4.1.0 to
v4.1.3 by restarting varnishd on a server. I can see that inode of the
executable and libraries are right for the updated files in the running
varnishd via /proc/<pid>/maps (on Linux), I can see that no other varnishd
processes are running. I expected that restarting varnish would cause the
cache to be wiped and require some period of warmup, however the hit rate
held steady at 98% and I don't know why.

I'm very happy to be able to do this, its going to make the rest of my
upgrades non-disruptive, but I'd like to be able to explain whats happening
behind the scenes. Can anyone tell me how the cache seemed to survive the
restart?
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