A behavioural model built against a bank's existing workbook has to tally — the percentage changes, the percentiles, the regression coefficients, the final core — to several decimal places before it is accepted. Only once it reconciles do two things go beyond a standard build: a SARIMAX version that handles the autocorrelation a plain regression misses, and walk-forward backtesting that scores the model out-of-sample before it is relied on. Reconcile to the numbers first, then improve on them — that is what a validation reviewer wants to see.
Plain-English write-ups, methodology only, on illustrative data.
All figures illustrative — methodology only, not any client's data or code.