pfSense RRD graphs for NTP - system jitter vs clock jitter

Since installing pfSense and setting up the NTP server, I've been wondering for a while what the difference is between "System Jitter" (sjit) and "Clock Jitter" (cjit) in the RRD graphs.  For instance, in the following graph, we can see that the system jitter value has gone way up.

So what causes system jitter? Looking at the Status -> NTP page gives us a bit of a clue.

At least two of our upstream time servers have huge jitter numbers, 20-40ms, while all of the other upstream time clocks are reporting jitter of less than 1ms.  So something is happening to those two time servers, or a common route between us and those time servers.

That leads me to believe that the green line in the NTP graphs (sjit) indicates how much jitter there is between our NTP server and the upstream servers.  In general, it probably means there is congestion between us and that server and that packets are arriving out of order or late.