Production is different. If you use water-wash flux, or use a soponifier to make rosin flux water soluble, you've got to remove a lot of conductive stuff. Keep doing that and you contaminate your wash water. So first-stage wash is dirty. Last-stage wash needs to be kept very clean and get under all the surface-mount parts. People screw this up a lot a Tohm leakage levels.
We recently got some relays that were imperfectly sealed, and the boards were assembled by a contractor. First-stage water wash pushed conductive crud into the relays, and subsequent washes didn't pull it out.