If the console has just been flipped to a VT, X is already running. Running startx would attempt to start it again. VTs are a kernel feature. The X terminal emulators use them, but most Linux distributions also set up 4-5 console VTs as well. The "real" terminal is the physical computer screen and keyboard.
edit: actually, I think I was wrong about the "real terminal" being the screen. The terminal model hails from the days of massive multiuser systems, when a single computer had to support tens or hundreds of terminals (the original ones were called "teletypewriters", which is where the abbreviation "tty" and the error message "not a typewriter" come from) connected via serial ports or similar connections. So I think it might be more accurate to say that there is no "real terminal" on desktop systems, and the kernel is doing terminal emulation through VGA and the keyboard controller. I guess this is mostly a semantic thing, though. And I just wrote a rant on it longer than my original post. Damn it.