If the data doesn't support that - and I haven't the time to analyze the analysis to determine if it doesn't - it's probably due to actions TfL take to counter it. Buses don't stop as often to "even out gaps in the service" as the tube does, but standing at bus stops, I frequently see buses "disappear": two buses look like they're going to arrive at the same time, but one of them turns into a different bus, like it's been reallocated.
1) The "first bus collects delay" theory is responsible for a part of the difference between the data and the model, after all there is some.
2) There is another mechanism at action that statistically reverses the effect of the "first bus collects delay" theory, not necessarily in the same bus runs.
Number 1) is coherent with Occam's razor.