Block time is only designed to average every 10 minutes.  If difficulty spikes, a few blocks will be quicker.  As difficulty drops, a few blocks will be longer.  That's just the lag inherent in the difficulty calculation, to keep wild swings in check.

At present, it's trending upward, with some noticeable dips along the way -- so some block times will be greater than 10 minutes, and some less than.