为什么groupby（）.sum()后日期不在列名里

import numpy as np
import pandas as pd


data1 = pd.read_csv('result.txt',sep='\t')
data_weather = pd.read_excel('2017-2020北京天气数据2（包含气压）.xlsx')
data_weather = pd.DataFrame(data_weather)
data_weather = np.array(data_weather)
data1 = pd.DataFrame(data1)
data1 = data1['用气量'].groupby(data1['日期']).sum()
data1 = pd.DataFrame(data1)
print(data1.head(5))

后面加个.reset_index() data1 = data1['用气量'].groupby(data1['日期']).sum().reset_index()