Why does every metric come back with a window attached?

Every figure this API returns carries the span it covers, because a rate measured over seven days and one measured over ninety are not comparable and nothing in the number itself says which you have.

The window also reports latest_data_day — the most recent day with data for your current model and location filter. That is what makes a window that looks shifted explicable, and it tells you how fresh the data actually is.

The IAB’s reproducibility criterion asks providers to define acceptable variation within a 7-day window and to state confidence. Reading the window is the first step of that check.