I now attribute this drop to Gentoo's developer population returning toward an equilibrium after an explosive, uncontrolled growth. The Gentoo structure and governance could not scale quickly enough to deal with all the new developers, but it took some time to normalize and continues to do so.
Um, maybe I'm not being very bright, but isn't the most likely explanation for this part of the chart simply that you don't have a list of future commits yet to happen?
The reason I thought this was because of another chart with the same characteristic shape I've seen. The Potaroo automated address exhaustion site includes a chart of IPv4 address allocation vs usage against time, and whenever you look at it, there appears to be a very low ratio of usage for the last few months, which would suggest a disaster in Internet governance. But in reality if you look at the same period on the chart in a year's time the anomaly vanishes, because by then those allocated addresses are in use, it just takes time after they're allocated for that to happen.
I suspect that the same thing is wrong with your chart. Some of the people who haven't committed in July, August or September will pop up in November and thus retrospectively make the chart's "lifetime" indication wrong.
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