It's not the FDA that differentiates, it's the sponsor. They intentionally file as a brand as a business strategy.
It’s not always the sponsor’s intent to have a non-substitutable product. Duramed’s Cenestin, which was the subject of a recent thread on this board, is a case in point. The FDA rejected the ANDA and told Duramed to resubmit under 505b2.
“The efficient-market hypothesis may be the foremost piece of B.S. ever promulgated in any area of human knowledge!”