"joinLeft" returns dataframe that smaller than left_df

Hi all:

I found that **leftJoin** generates df that smaller than the left df

```
[In] [1]: joined_flint = left_flint.leftJoin(right_flint, tolerance=tolerance, key=by)
[In] [2]: print (joined_flint.count() < left_flint.count())
True
```

I consider this is a false result since left join does not drop any row in the left table.
Any explanation or suggestion?