Late response, so don't know if this question is still relevant for you, but the engine uses some heuristics to choose how to behave when selections are made. I don't know if this is formally documented anywhere but, but I think the heuristics are something like this:
A selection of only "possible" values adds the newly selected values to the total set of selected values.
A selection of only "excluded" values clears a part of the selection state so that the newly selected value is no longer excluded. The exact algorithm for choosing what to clear is unknown to me.
A selection of both "possible" and "excluded" values at the same time will be similar to 1. with the difference that the extra "excluded" values will have no effect on the selection state. They will be remembered though, so if you later clear the selection of a field that makes the "excluded" value "possible" again, then it will immediately be treated as "selected".
I can recommend creating an app with the Ctrl+00 script and play around with it. Add a filter pane with for instance the dimensions Dim1, Dim2, Dim3 and Alpha to see how it behaves with different combinations of selections.