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.

Did they do any experimentation with pre-caching new content? What percentage of those new photos each day end up being requested I wonder.

I'm not sure if Wyatt, Qi, et al. looked at pre-caching at all. I've not looked at the numbers yet, but it's on my list of things to look into once the next few months of more obvious wins are worked through.