そうみたいですね。PURGEも調べてみたらSquidの資料ですがこんなことが書かれていました。

> Squid accepts a custom request method for removing cached objects. The PURGE method isn't one of the official HTTP request methods. It is different from DELETE, which Squid forwards to an origin server. A PURGE request asks Squid to remove the object given in the URI.

言われてみればなるほどという感じですね。

http://etutorials.org/Server+Administration/Squid.+The+definitive+guide/Chapter+7.+Disk+Cache+Basics/7.6+Removing+Cached+Objects/