Same reason Apple made up random image names for bookmark icons on the homescreen on the iPad and now they pollute our logs.
The whole favicon retrieval seems to be a mess. I wish the W3C draft about manifests for web apps gets wide support and can be used on normal sites, too. (wishful thinking...?)
http://w3c.github.io/manifest/
Same reason Apple made up random image names for bookmark icons on the homescreen on the iPad and now they pollute our logs.