It's luck, nothing more. Goalies don't sustain this throughout their careers.
I'm curious if there has been a test for this. Is it possible to look at Marty's Corsi or shots per game (or whatever similar metric) vs. various backup goalie metrics?
Here are the Shorts For/Against % (shots Devils took divded by shots opponents took)
Year Marty Backup Diff
2013-14 0.512 0.531 -0.019
2012-13 0.547 0.532 +0.015
2011-12 0.501 0.505 -0.004
2010-11 0.532 0.499 +0.033
2009-10 0.513 0.533 -0.020
2008-09 0.502 0.555 -0.053
*Interestingly enough, we seem to be carrying the play more when Schneider is in net, at least as far as shot breakdown goes. When Schneider is playing we average 26 shots on net and 23 against. Compared to Brodeur with 25 shots on net and 24 against.
Note: score effects causing this? Chicken and the egg