My understanding is that the builder's plates with numbers at Baldwin Locomotive Works appear to have been applied at some point (completion?) in the manufacturing process. With many orders at the factory at one time there would be a good chance that an order would not have consecutive numbers. However, the complete class number -- which is more like a serial number -- would be consecutive within an order.
American Locomotive was different. It assigned construction numbers when the customer's order was entered into the records.
BN