Can anyone tell me or no why when a customer meaning an airline orders say 10 B747’S OR 5 B789 aircraft from the plane maker. After they plane comes out or off the final line and is painted into that airlines colours. Why is the rego still showing the Boeing company and not the airline who is having it delivered. I noticed this many of times when they are out testing the aircraft on short flights touch and go or doing there tests at high altitudes. Then on the day the plane leaves to its new owners the new rego is in place. Can anyone tell me why this occurs 
Where are the test flights being done?
That’ll answer your question.
Look at your post again. You answered it in the penultimate (second to last) sentence.
It doesn’t have to do with where the tests are being done.
The aircraft belongs and is registered to Boeing until it is turned over to the airline. Then that airline (or leasing company) puts its own registration on the aircraft.
Boeing, by the way, reuses registratins. I believe they have an agreement with the FAA to do this so they don’t have to have a lot of different registrations.
What he said.