rsync "speedup"

[Harry Putnam]

People here are up on the various transfer styles, so hope someone
won't mind answering this question:

Using rsync (not concerned here with its shortcomings under rcp.el)
one always gets a message about "speedup":
Speedup is 167.27  or something similar.  Interpreting this is not
documented in rsyncs manpages.  Only a general reference that rsync
speeds up file transfer.

Anyone here know what that line actually refers to?