Because Chromium, on which Iron is based, runs with the same WebKit rendering engine as Safari.
When Chrome/Chromium was developed, since it was a new browser, one of the stated design goals was that Chrome/Chromium should render webpages the same as Safari/WebKit. The "Safari" in the user-agent allows for old/non-updated websites that check the user-agent for "Safari" to allow Chrome/Chromium/Iron to (hopefully) be detected and function the same as Safari.
OK thanks, that makes a lot of sense. I've always liked Webkit based browsers, Safari 3 was quite good & I liked it a lot despite some of its incompatibilities. I'm not a great fan of Safari 4. It's so heavy & power hungry I think it would only work well on the Starship Enterprise LOL!