USING NETWORK FAILURES:
If you chose "synchronous: fail", then when the link is broken, the Oracle application on the Primary site would get I/O errors and stop. Now following your procedure you should have the same data on both sides.
If you chose "synchronous: override", then when the link is broken, the Oracle application on the Primary converts to asynchronous mode, and will flush data and be more "up to date" than the Secondary, and you are correct, they will be different.
If you chose "synchronous: off" then of course the Primary will always have newer data then the Secondary (since this is asynchronous mode).
USING SITE FAILURES:
Note that if you had "synchronous: override" (or "synchronous: fail") and the Primary site CRASHES (not a network issue), then the secondary would have no data loss, and the data on the Secondary would be the same as what the Primary had when it failed.