Twenty of the top 25 teams ranked first in DER. That makes sense, since if you turn alot of balls in play into outs, you will give up fewer runs (and fewer than expected based on walks, strikeouts and HRs, which figure into FIP ERA). The 1939 Yankees had a 106-45 record and the all-time best run differential of 411.
Teams might also see their actual ERA go below their FIP ERA if they pitch well with runners on base. But my post last week showed that although that matters, DER probably matters more.
Twelve of these 25 teams came between 1929 & 1940 (I wonder what the reason is). Many of the 25 were pennant winners or made the playoffs.
Four Giants teams from the 1930s are on the list. They led the NL in DER 4 times in the decade and were 2nd three times. They also led in fielding pct. 4 times with one 2nd. They had the highest DER for the decade of 0.7001 (2nd place had 0.6960). Maybe they are one of the great fielding teams in baseball history. But it is not anything I have ever heard about.