为什么这两段代码返回不同的输出？

Question

代码段1：

d = census_df[census_df.SUMLEV==50].copy()
d['max'] = d[['POPESTIMATE2010','POPESTIMATE2011','POPESTIMATE2012','POPESTIMATE2013','POPESTIMATE2014','POPESTIMATE2015']].max(axis=1)
    d['min'] = d[['POPESTIMATE2010','POPESTIMATE2011','POPESTIMATE2012','POPESTIMATE2013','POPESTIMATE2014','POPESTIMATE2015']].min(axis=1)
    d['diff'] = d['max'] - d['min']

d[d['diff'] == d['diff'].max()].iloc[0]['CTYNAME']

代码段2：

d = census_df[census_df["SUMLEV"] == 50]
d= d.groupby(["STNAME"])["POPESTIMATE2015'].nlargest(3)
d["max"] = d[['POPESTIMATE2010','POPESTIMATE2011','POPESTIMATE2012','POPESTIMATE2013','POPESTIMATE2014','POPESTIMATE2015']].max(axis=1)
d["min"] = d[['POPESTIMATE2010','POPESTIMATE2011','POPESTIMATE2012','POPESTIMATE2013','POPESTIMATE2014','POPESTIMATE2015']].min(axis=1)
d["Diff"] = d["max"] - d["min"]
ans = d[d["Diff"] == d["Diff"]].max().iloc[0]["CTYNAME"]