Inconsistent results between 'nlargest'  and 'sort_values' function
```python
df = pd.DataFrame({'a': [-2, -1, 1, 10, 8, 11, -1],
       'b': list('abdceff'),
       'c': [1.0, 2.0, 4.0, 3.2, np.nan, 3.0, 4.0]})    

df
Out[316]: 
    a  b    c
0  -2  a  1.0
1  -1  b  2.0
2   1  d  4.0
3  10  c  3.2
4   8  e  NaN
5  11  f  3.0
6  -1  f  4.0

df.nlargest(5, ['a', 'c'])
Out[317]: 
    a  b    c
6  -1  f  4.0
5  11  f  3.0
3  10  c  3.2
4   8  e  NaN
2   1  d  4.0

df.sort_values(by=['a','c'], ascending=False).head(5)
Out[318]: 
    a  b    c
5  11  f  3.0
3  10  c  3.2
4   8  e  NaN
2   1  d  4.0
6  -1  f  4.0
```
I think their results should be the same.