Ric Finlay did the comparison on a ten-year basis.I had a look at the amount of Shield centuries being scored during the past 20 years and there appears to be a lower amount of centuries being scored since the Big Bash started. Last year was the lowest number of centuries scored in the past 20 seasons (52). Highest was 70 twice.
What was noticeable was players scoring 5-8 centuries in a shield season. This appears to be much rarer lately. These days 3 will be enough to score the most in a season. Gone are the days of Bevan (8), Elliott (7), Hayden (7), Lehmann and Katich (6). Voges in 14/15 with 6 is pretty much the only player since early-mid 2000's to score 6 or more. Rogers is the last player with 5 in 08/09. There is just no consistency lately.
http://stats.espncricinfo.com/ci/engine/records/batting/most_runs_series.html?id=114;type=trophy
1997-98: 70
1998-99: 68
1999-00: 67 (6 by Lehmann)
2000-01: 57 (6 by Katich)
2001-02: 66
2002-03: 58
2003-04: 69 (7 by Elliott)
2004-05: 65 (8 by Bevan)
2005-06: 62
2006-07: 60
2007-08: 60
2008-09: 68
2009-10: 63
2010-11: 55
2011-12: 58
2012-13: 58
2013-14: 57
2014-15: 70 (6 by Voges)
2015-16: 64
2016-17: 57
2017-18: 52
I think they call that reversion to the mean.