The existence of one plausible explanation (good model fit) does not automatically invalidate another plausible explanation (mechanism of action).
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.