Pandas MultiIndex levels() and get_level_values() give different results

Code Sample, a copy-pastable example if possible

import pandas as pd

raw_data = {'regiment': ['Nighthawks', 'Nighthawks', 'Nighthawks', 'Nighthawks', 
                         'Dragoons', 'Dragoons', 'Dragoons', 'Dragoons', 'Scouts', 
                         'Scouts', 'Scouts', 'Scouts'], 
            'company': ['1st', '1st', '2nd', '2nd', '1st', '1st', '2nd', '2nd', 
                        '1st', '1st', '2nd', '2nd'], 
            'name': ['Miller', 'Jacobson', 'Ali', 'Milner', 'Cooze', 'Jacon', 
                     'Ryaner', 'Sone', 'Sloan', 'Piger', 'Riani', 'Ali'], 
            'preTestScore': [4, 24, 31, 2, 3, 4, 24, 31, 2, 3, 2, 3], 
            'postTestScore': [25, 94, 57, 62, 70, 25, 94, 57, 62, 70, 62, 70]}

df = pd.DataFrame(raw_data, columns = ['regiment', 'company', 'name', 'preTestScore', 'postTestScore'])
df_gbm = df.groupby(['regiment', 'company']).mean()
df_gbm_preTS_gt10 = df_gbm[df_gbm.preTestScore > 10]

df_gbm_preTS_gt10.index.levels[0].tolist()
df_gbm_preTS_gt10.index.get_level_values(0).unique().tolist()

Problem description

I was expecting MultiIndex.levels[0].tolist() to give the same results as MultiIndex.get_level_values(0).unique().tolist() but that is not the case.

df_gbm_preTS_gt10

		preTestScore	postTestScore
regiment	company
Dragoons	2nd	27.5	75.5
Nighthawks	1st	14.0	59.5
	2nd	16.5	59.5

df_gbm_preTS_gt10.index.levels[0].tolist()
['Dragoons', 'Nighthawks', 'Scouts']

df_gbm_preTS_gt10.index.get_level_values(0).unique().tolist()
['Dragoons', 'Nighthawks']

Expected Output

df_gbm_preTS_gt10.index.levels[0].tolist()
['Dragoons', 'Nighthawks']

Output of `pd.show_versions()`

INSTALLED VERSIONS commit: None python: 3.6.1.final.0 python-bits: 64 OS: Linux OS-release: 4.10.0-20-generic machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: en_US.UTF-8 pandas: 0.19.2 nose: None pip: 9.0.1 setuptools: 27.2.0 Cython: None numpy: 1.12.1 scipy: 0.19.0 statsmodels: None xarray: None IPython: 6.0.0 sphinx: None patsy: None dateutil: 2.6.0 pytz: 2017.2 blosc: None bottleneck: None tables: None numexpr: None matplotlib: 2.0.0 openpyxl: None xlrd: None xlwt: None xlsxwriter: None lxml: None bs4: None html5lib: 0.999 httplib2: None apiclient: None sqlalchemy: None pymysql: None psycopg2: None jinja2: 2.9.6 boto: None pandas_datareader: None

Comment From: jreback

you have unobserved levels which persist thru indexing

see http://pandas-docs.github.io/pandas-docs-travis/generated/pandas.MultiIndex.remove_unused_levels.html?highlight=remove_unused#pandas.MultiIndex.remove_unused_levels

and the original issue: https://github.com/pandas-dev/pandas/issues/2770

Comment From: miishke

OK, thanks. Should I make a PR for documenting this? Because this is the entry for pandas.MultiIndex.levels in the API documentation:

selection_001