The prototype took three weeks
The prototype took three weeks. The production system is quoted at nine months. Owners read those two numbers and conclude someone is lying.
Nobody is. The prototype and the product are different objects that happen to look the same on a screen.
The prototype handles the happy path for one user with clean data. Production handles the customer who enters their phone number in the address field, the integration that returns nothing on Tuesdays, the manager who needs to undo what a clerk did last week, and the auditor who wants to know who changed the price.
The prototype has no permissions, because everyone in the demo is an admin. Production has to know who may see margins.
The prototype was never migrated. Production has to swallow eleven years of records from the old system, including the ones that are wrong.
AI coding tools have made this gap wider, not narrower. The three weeks is now three days. The nine months barely moved, because the nine months were never about typing.
So when the gap surprises you, do not argue about the nine. Ask what is in it. A good team can list the items: migration, integrations, permissions, exceptions, testing, training. A bad team says "hardening." If the list is real, the number is real. If the number arrived before the list, it is a guess with a margin built in, and you are the margin.