It's part of the WebKit UA. It harkens from the days when IE and Netscape were the only browsers developers thought about when UA sniffing. It made sure WebKit got the Netscape version (which was closer to the standards), not the IE version.

It's the same reason all browser UAs start with "Mozilla" even though only one is actually made by Mozilla.

It's in every browsers' User Agent. I think it's code for "not ancient IE".

All browsers, desktop or mobile, have "like Gecko" as of now.

Konqueror/KHTML has been doing this since at least Konqueror 3.2:

http://www.zytrax.com/tech/web/browser_ids.htm#kong