If you look at the Rams' and Seahawks' schedules over the next three weeks, they are pretty similar.
Both have one game they really should win (Rams@Arizona, Seahawks@49ers) and two that will be very tough (Rams vs. Saints, Rams vs. Eagles, Seahawks vs. Eagles, Seahawks@Jaguars).
It is entirely possible that both teams will win only one of the next three. If that happens, they would be 8-5 (Rams) and 7-6 (Seahawks) when they meet. If that happens, a Rams win would almost certainly mean the NFC West Title, while a Seahawks win would mean that they'd be tied in record with the Seahawks owning the tiebreaker. If that happens, of course, the Rams would need help in the final two weeks to win the division.
So...
Nothing has been decided yet!