Assume two projects PRA and PRB, both with the category "somecat".
If a board has the filter
category = somecat
it will not show up in either PRA or PRB.
However, if the filter is changed to
project in (PRA,PRB)
the board will show up in both projects.
It's important to note that the board itself works properly, all issues in PRA and PRB would be matched by the filter, the issue is only that the board wouldn't be accessible from either project.