That's on purpose, of course, so that you can't vote again (at least not for some time). It wouldn't make much sense if you could vote as often as you liked, don't you think?
Geeklog uses a combination of a cookie and your IP stored in the database to prevent you from voting again for a certain amount of time (see variables $_CONF['pollcookietime'] and $_CONF['polladdresstime'] in config.php - they hold those times, in seconds).
Of course, this system - like any - can be circumvented, so you shouldn't be expecting 100% correct results from a vote. But it seems to help a bit, as you just demonstrated ;-)
bye, Dirk