I been work in an OEM company b4. Doing some Branded phone. First released phone have the best quality in term of cosmetic, and functional. We call it Grade A product, to ensure user have great experience. The grade B and C will keep as stock. When large order come in, and production unable to cover, they will start release B and C grade
Some error or design flaw will only found after launch the phone. So they will have revise version on PCB or design.