BUG: in DataFrames(), parameters columns and index have some problems
```python
from scipy.io import netcdf
import numpy as np
ncFileName = 'sst_mask_cutlatlon.nc'
gridCSVName = 'mask_grid.csv'
f = netcdf.netcdf_file(ncFileName, 'r')
lon = f.variables['lon'].data # 251
lat = f.variables['lat'].data # 331
mask = f.variables['mask'].data[0] # (331, 251)
newMask = np.where(mask == 1, mask, 0)
pd.DataFrame(newMask, columns=lon, index=lat).to_csv(gridCSVName)
f.close()
```
#### Problem description

when I print(lat), the ouput is
```python
[21.225 21.275 21.325 21.375 21.425 ... 37.575 37.625 37.675 37.725] # len == 331
```
but if a print(pd.DataFrame(newMask, columns=lon, index=lat)), the index is:
```csv
21.2250004
21.2749996
21.3250008
21.375
21.4249992
21.4750004
21.5249996
21.5750008
21.625
...
```

why will this happen?


<details>

the nc file(89kb) is in this url: https://pan.baidu.com/s/1fJk6QYiETWlCj_bTO_xShQ

my environment is:
macOS Sierra and python 3.4.3

</details>
