The decomposition of the difference between a run rate, or the prior forecast, and the current forecast into named components: new deals, expansion, slipped, pushed, lost.
Take every deal in the pipeline history with its stage, close date and value at two points in time. Classify the change: new since the prior snapshot, moved in from a later period, moved out to a later period, closed, lost, resized. Sum each class. The classes bridge the two totals exactly.
"Q4 is $1.4m above run rate" is a claim. "Three deals explain $1.1m of it, all pushed from Q3, all having slipped twice already" is a bridge, and it tells the CFO what to believe.
A bridge needs stage history, not a snapshot. Two quarters of history gives a bridge; two years gives slip patterns by rep.
Starting from the prior forecast or the run rate, named lines add and subtract to reach the current forecast: deals won, lost, slipped out, pulled in, resized, and new. The lines sum exactly to the difference.
Last month's forecast was $13.3 million. Won early plus $0.2 million, slipped out minus $0.7 million, lost minus $0.3 million, new deals plus $0.4 million. This month's forecast: $12.9 million.
A forecast that changes with no explanation of which deals moved. Trust in the number goes with it.