You start your own RSN so you can collect carrying fees from the cable companies (based on the number of subscribers with MASN in their cable package) and then pay a lower rights fees to the team (i.e. yourself) to show the games. The difference between the two numbers isn't subject to revenue sharing.
Without the Nats, it seems like the O's TV territory would have been the exact same, so I don't see where the extra revenues from adding the Nats would have come from. And with the Nats, you not only have to pay rights fees to two teams, you also have to pay higher rights fees because of the Nats' lawsuit. So not only do the fees paid to the Nats come out of your profits, the increased rights fees that you pay to yourself are now subject to revenue sharing. Both things are bad.
One way the revenue increased was like this. When HTS/Comcast paid the O's, those channels were additional fees that consumers paid, because they wanted to watch baseball. With MASN, Anglos dictated that every subscriber would pay a MASN premium, whether they wanted to or not. Direct TV agreed, Comcast didn't. So Angelos told they they had two choices, charge every subscriber, or you can't air my new RSN. They had no choice but to concede.
Also, two MASN channels = twice the commercial time they can sell.