Could not tell you. Could be a large variety of things.
This is a test using the current example configs shipped at each date - which means it's not always apples to apples if default config changes. Analysis could have changed for our default english text. New defaults for features or ease of use may have been enabled.
For example, I believe the update log is on by default now for durability and realtime GET, etc.
Also, some code paths have changed to support various new features.
Also, Lucene is changing underneath us, so we should probably compare to some similar benchmark there (I know Mike publishes quite a few that could be looked at).
It's not so easy to dig in after the fact with month resolution.
At some point, it would be nice to have this automated and published as Lucene is - then we could run it nightly.
There is some work to do to get there though (don't know that ill have time for it in the near future), and we would need a good consistent machine to run it on (I could probably run it at night or something).
I have not attempted to track anything down other than the broad numbers right now.
This is simply to start a record that can help as we move forward in evaluating how changes impact performance.
Obviously the single threaded path has not been affected - so whatever has changed, it's likely mostly around concurrency.