BUG: groupby resample different results with .agg() vs .mean()
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

#### Code Sample, a copy-pastable example

```python
import pandas as pd
import numpy as np

data = pd.DataFrame({
    'cat': ['cat_1', 'cat_1', 'cat_2', 'cat_1', 'cat_2', 'cat_1', 'cat_2', 'cat_1'],
    'num': [5,20,22,3,4,30,10,50],
    'date': ['2019-2-1', '2018-02-03','2020-3-11','2019-2-2', '2019-2-2', '2018-12-4','2020-3-11', '2020-12-12']
})
data['date'] = pd.to_datetime(data['date'])
using_agg = data.groupby('cat').resample('Y', on='date').agg({'num': 'mean'})
using_mean = data.groupby('cat').resample('Y', on='date').mean()

print(using_agg)
print(using_mean)

assert(np.allclose(using_agg['num'], using_mean['num']))

```

#### Problem description

I expect to get the same result from using .agg({col_name: 'mean'}) and I expect to get from .mean()

It's very surprising the results are different here, and really worrying for me, considering historic code for us might be producing incorrect results. Can anyone shed some light on why this may be the case?

#### Expected Output

The output from `using_mean` is correct and should be equal to the output from `using_agg`

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.6.8.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 19.3.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.3
numpy            : 1.18.2
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 18.1
setuptools       : 40.6.2
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None

</details>
