DataFrame resample and Grouper ,Why are the results different?

[CrystalWindSnake]

Code Sample, a copy-pastable example if possible

data:

idx = pd.date_range('2017-1-1','2018-12-31',freq='M')
df = pd.DataFrame(np.arange(len(idx)),idx)

resample:

df.resample('3m',kind='period').sum()

use grouper:

df.groupby(pd.Grouper(level=0,freq='3m',kind='period')).sum()

Problem description

Why are the results different?

Expected Output

I hope that the result of using the grouper is the same as the resample method.

thank you!!

Output of pd.show_versions()

INSTALLED VERSIONS

commit : None
python : 3.7.3.final.0
python-bits : 64
OS : Windows
OS-release : 10
machine : AMD64
processor : Intel64 Family 6 Model 58 Stepping 9, GenuineIntel
byteorder : little
LC_ALL : None
LANG : None
LOCALE : None.None

pandas : 0.25.0
numpy : 1.16.2
pytz : 2018.9
dateutil : 2.8.0
pip : 19.0.3
setuptools : 40.8.0
Cython : 0.29.6
pytest : 4.3.1
hypothesis : None
sphinx : 1.8.5
blosc : None
feather : 0.4.0
xlsxwriter : 1.1.5
lxml.etree : 4.3.2
html5lib : 1.0.1
pymysql : None
psycopg2 : None
jinja2 : 2.10
IPython : 7.4.0
pandas_datareader: None
bs4 : 4.7.1
bottleneck : 1.2.1
fastparquet : None
gcsfs : None
lxml.etree : 4.3.2
matplotlib : 3.0.3
numexpr : 2.6.9
odfpy : None
openpyxl : 2.6.1
pandas_gbq : None
pyarrow : 0.13.0
pytables : None
s3fs : None
scipy : 1.2.1
sqlalchemy : 1.3.1
tables : 3.5.1
xarray : None
xlrd : 1.2.0
xlwt : 1.3.0
xlsxwriter : 1.1.5

[WillAyd]

This is probably related to #28302 if you'd like to investigate and submit a PR I think would certainly be welcome