-- Search Engine Spider and User Agent Identification
JD_Toims - 12:30 am on Aug 25, 2013 (gmt 0)
Sometimes it uses HEAD HEAD /payb.php/downloads/abc/2013/abc.tar - HTTP/1.1 google-xrawler
Google has stated they do not use HEAD requests, because in their testing it really didn't provide any faster spidering since they had to make two requests rather than one for any modified page request and when they're just grabbing HTML it's a very fast process to just use GET.
It may be they've started using HEAD for grabbing something like a .tar, but personally I'd guess it's a spoof until proved otherwise.