They are the only team in the top 25 since 1981 (also the only team in the top 50 since 1981). This is probably due to their fielding (they lead the NL in DER with .732). Top 3 teams since 2006 in defensive efficiency rating are the 2016 Cubs .732, 2011 Rays .724, 2013 Reds .715. Cubs might be highest going even farther back but I stopped checking at 2006.
There is usually a pretty high correlation between BAbip allowed and DER for all MLB teams. Here is the correlation in some years
But it was only -.73 in 1913. It was -.89 in 1914. I don't know why it was so low in 1913.
It might be worthwhile to do BAbip relative to the league average. It looks like the other teams on the list are from years that will have a relatively low league BAbip. The 2016 NL is 16th highest since 1913.
Update 9-20-2016: The Cubs are .046 below the league average and that appears to be the best ever (although I was having a bit of a problem for some years in finding the league average using the Excel VLOOKUP tool-if anyone is good at that, please comment here or email me)
Here are the best teams (I think I solved the VLOOKUP issue). Let me know if this formatting does not work. The third number is that team's BAbip allowed, then the league average and then the difference.
Why are the Cubs doing so well? Fielding? Something about their pitchers? Park effects? Just luck?
NYY 1939 0.256 0.293 -0.037
SEA 2001 0.262 0.297 -0.035
LAD 1975 0.25 0.284 -0.034
CIN 1999 0.267 0.300 -0.033
OAK 1980 0.258 0.288 -0.03
OAK 1990 0.258 0.287 -0.029
BRO 1941 0.249 0.277 -0.028
BAL 1979 0.258 0.286 -0.028
CHW 1991 0.26 0.288 -0.028
DET 1983 0.258 0.286 -0.028
NYY 1919 0.261 0.289 -0.028
Here is a comment someone left at Fangraphs. See The Most Extraordinary Team Statistic by Jeff Sullivan
"When I saw this headline, I was hoping you’d land on the Cubs’ ability to squash balls in play. But how about these other options for ways to quantify it?
BIP-Wins. Cubs are #1 all-time (since 1871):
…and they have derived 9.5 more wins from this ability than the #2 team this season:
BP’s Park-Adjusted Defensive Efficiency. The 2016 Cubs are #1 by a wide margin, but this stat only goes back to 1950:
OPS+ allowed. B-R only has solid data going back to 1923 for this stat. During that period, the 2016 Cubs are #1 by three percentage points: