nth() mixes column order

[sursu]

Consider the following dataframe:

df = pd.DataFrame([[179293473,'2016-06-01 00:00:03.549745','http://www.dr.dk/nyheder/',39169523],[179293473,'2016-06-01 00:04:22.346018','https://www.information.dk/indland/2016/05/hvert-tredje-offer-naar-anmelde-voldtaegt-tide', 39125224],
 [179773461, '2016-06-01 22:13:16.588146', 'https://www.google.dk', 31658124],
 [179773461, '2016-06-01 22:14:04.059781', 'https://www.google.dk', 31658124],
 [179773461, '2016-06-01 22:16:37.230587', np.nan, 31658124],
 [179773461, '2016-06-01 22:23:09.847149', 'https://www.google.dk', 32718401],
 [179773461, '2016-06-01 22:23:55.158929', np.nan, 32718401],
 [179773461, '2016-06-01 22:27:00.857224', np.nan, 32718401]],
columns=['SessionID', 'PageTime', 'ReferrerURL', 'PageID'])

which looks like this:

SessionID	PageTime	ReferrerURL	PageID
179293473	2016-06-01 00:00:03.549745	http://www.dr.dk/nyheder/	39169523
179293473	2016-06-01 00:04:22.346018	https://www.information.dk/	39125224
179773461	2016-06-01 22:13:16.588146	https://www.google.dk	31658124
179773461	2016-06-01 22:14:04.059781	https://www.google.dk	31658124
179773461	2016-06-01 22:16:37.230587	NaN	31658124
179773461	2016-06-01 22:23:09.847149	https://www.google.dk	32718401
179773461	2016-06-01 22:23:55.158929	NaN	32718401
179773461	2016-06-01 22:27:00.857224	NaN	32718401

Run:
df.groupby('SessionID').nth(-1)

Out:

SessionID	PageID	PageTime	ReferrerURL
179293473	39125224	2016-06-01 00:04:22.346018	https://www.information.dk/
179773461	32718401	2016-06-01 22:27:00.857224	NaN

Question: Why has nth() mixed the order of my columns?

[ron819]

@mroeschke @jreback
Not sure if this is a bug but it would be nice if the nth() won't change the orders of the columns.