Sprint Burndown Chart - Details on time logged metric?

[Tom Cooper]

I am using "Remaining Time Estimate" as the chosen metric on my burndown report, so I get a red line which burns down the hours estimated for tasks in the sprint.  I also get the green line, which is supposed to report the time spent.  

My users are entering time as we would expect, updating remaining hours and zero'ing out (sub)tasks that are completed ahead of schedule.  The red line reflects exactly what I would expect.  

I can't figure out the logic behind the green line though.  From what I can see it updates on some sort of schedule, usually at 7:00PM(00:00 GMT), but not always. For instance in the image I'm attaching, on 4/17 it jumps up at 9:00AM.

I've also noticed sometimes it doesn't take work logged into account even when it does jump up.

My questions are:

• What causes the time logged to update?
• What causes some worklogs, but not others against items on the board to be reflected?