While points difference can come into it, there are bonus points awarded that go to the main points total before points difference come into it. Like this : A win is 4 points, a draw worth 2. Loss worth 0 (obv.) score 4 tries and over is worth an extra point and also if you lose by less than 7 points you'll get a point for that too. So as these points count first before points difference.
The BBC gave this permutation:
If Scotland lose to England without a losing bonus point they are out of the World Cup
If Scotland lose to England with a losing bonus point and Argentina lose against Georgia, Scotland go through
If Scotland draw with England, they need Argentina to lose against Georgia
If Scotland and Argentina both win without a bonus point, it will come down to points difference between the three teams, though England are way out in front
If Scotland and Argentina win with bonus points, and England do not earn a losing bonus point, England are out.
It used to be a lot simpler when I was still playing Rugby :(