Yes, mostly due to new content. You have to miss on the first request for the new content, and each of the 350 million or so photos a day will contribute a miss. Also, this only ran over a particular time period, so one-off requests for older content (someone revisiting their old photo albums) would be misses too.

I'm not sure whether a "refresh" (ie, 304 not modified due to expiration) is counted as a miss in this data.