I maintain two accounts on OS X: one for software installation & updates, and the other being my primary account with limited credentials. That way, if I were to accidentally run some malicious code, it wouldn't hose the entire system.

So Google Chrome.app resides in /Applications, but it's installed by the account that gets created when you reinstall OS X (which is by default an account with full privileges). Standard users can read /Applications, so they can launch Google Chrome.app, but the process can't write to /Applications because of its effective UUID. That's why it prompts for an admin password.