The prior you set six months ago is not the prior for today. Registers tighten, capital-account regimes soften, entire categories of counterparty quietly move from ordinary to interesting or the reverse. Half the calibration work in this discipline is noticing that the ground underneath a base rate has moved, and re-anchoring before the reports built on the old anchor start being wrong in a way nobody catches for a quarter.
We revisit the base rates on a fixed schedule. Not because we expect them to change every quarter, but because we do not want the review to be triggered by drama. Drama drives over-correction; a scheduled review drives small adjustments.
The revision is boring; most base rates move a percent or two. Occasionally one moves more, and when it does, it is usually because the underlying category has quietly changed shape. That is the signal worth surfacing to clients, not the number itself.
The team that owns base rates is small and the work is slow. Both are on purpose; a large, fast base-rate team would produce more revisions than the calibration actually warrants, and the churn would degrade the anchor rather than improve it.
A base rate is not a permanent feature of the world. It is a measurement of how often something happened over some window, and the window matters. A prior set six months ago in one country may already be wrong because the underlying rate has moved, not because the analysis was flawed.
For anyone trying to reason about probabilities in daily life, the discipline is the same: check whether the ground you are standing on is the ground you assumed. A lot of confident forecasting fails not at the arithmetic but at the assumption that the base rate is still the base rate.