Core updates should be stored in the cache directory

I ran wp core update for 3.9.1 => 3.9.2 on ~75 sites on my server.  
The upgrade files were not stored in .wp-cli/cache/core, and it appeared that a fresh download of the changed files were downloaded for each of the 75 sites.  

message in CLI was:
Downloading update from https://downloads.wordpress.org/release/wordpress-3.9.2-partial-1.zip
Unpacking the update...
Success: WordPress updated successfully

Re-tested after clearing .wp-cli/cache/core and no new files were placed there.