In the US, local news has historically been presented via an "affiliate" model, in which a local company is allowed to air their own programming in a particular region in exchange for a fee. So when you turn to the channel for NBC in los Angeles on the TV, it's from the national NBC most of the time but for local news it will switch to NBC Los Angeles, which is usually a different company. I would imagine NBC Los Angeles was originally an entirely different company than NBC national that later got bought by them.