Numerical errors in var(), std()
### In pandas:
```python
df = pd.Series([1,2])
df.var()
```
### Output
```python
0.25
```

### In numpy:
```python
np.var([1,2])
```
### Output
```python
0.5
```

#### Problem description
Why does Pandas Series.var() method return results by default without dividing by N? They don't agree with Numpy. And in mathematics, the results should also divided by N. And I am confused about the effect of the parameter ddof=0 option.

