Unexpected result when calculating standard deviation and variance
Pandas 0.8.1:

```
import pandas as pd
a=pd.Series([0, 1, 3, 6])
a.mean(), a.std(), a.var()
a.values.mean(), a.values.std(), a.values.var()
```

I would expect both last lines to return the same result. However, the former returned:

```
(2.5, 2.6457513110645907, 7.0)
```

and the latter:

```
(2.5, 2.2912878474779199, 5.25)
```
