First off, the state and tribes can and do agree to whatever they want. Or, p[erhaps more honestly, what even just one tribe wants as they do hang together well as opposed to the NT side who would rather cut each others' throats.
The other aspect which applies primarily to salmon because they are most successfully fished in salt, is that the 50:50 is supposed to count from the WA part of the Pacific on in. You could have lots more time in A10, but the coast would the ocean and the Straits.
At some point it would be nice if WDFW would very publically describe the tradeoffs they make. From the perspective of maximizing the quality of management (getting the number right) fishing just in the rivers on updated run sizes works best. For the fish. As we move out further from the river the "unknowns" in terms of run size, stock composition, daily catch, and so on are greater. If the escapement is important you have to adding buffers in case your estimates are off. Which they always are.
We are well past the point where there are few enough people trying to catch fish (on all sides) that risk is a real concern. Or should be.
There used to be, as part of Boldt, equitable adjustment which meant that the side that under harvested was paid back in fish. Perhaps, as in your example, imbalances resulting from A10 closures could be Pais back every 3 years, or 5, or some such.