The answer to this question we suggest is that the sample size is just too small to generalise.
Certainly the points margin in the last three finals (which might be considered the real pro era) has been a maximum of 9.
The data available on the editions to date is summarised below (source: World Rugby archives).
|RESULTS IN RWC FINALS TO DATE|
|1995||South Africa||New Zealand||3|
Also in graphic form here RWC Finals Points Difference