Logical comparison operators and mathematical operators are applied inconsistently for series.
In series all logical operators are by position (iloc) while all mathematical operators are by location (loc).
This is probably similar to https://github.com/pydata/pandas/issues/4581.

e.g.
`import pandas as pd`
`a = pd.Series([1, 4], index=[0, 1])`
`b = pd.Series([3, 7], index=[1, 0])`

`a>b`

Gives

`0    False`
`1    False`
`dtype: bool`

But 

`a - b`

gives

`0   -6`
`1    1`
`dtype: int64`
#### output of `pd.show_versions()`

`

> > > pd.show_versions()
## INSTALLED VERSIONS

commit: None
python: 2.7.10.final.0
python-bits: 64
OS: Darwin
OS-release: 15.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8

pandas: 0.18.1
nose: None
pip: 1.5.6
setuptools: 3.6
Cython: None
numpy: 1.11.1
scipy: None
statsmodels: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.5.3
pytz: 2016.4
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
boto: None
pandas_datareader: None
`
