ISO8601 vs POSIX offset clarification

Hi,

the documentation around how numeric offsets are parsed from strings is a
bit confusing, are they supposed to be treated as ISO8601 or POSIX ?

e.g.

select '2017-12-01 11:00:00 +11:00'::timestamp with time zone at time zone
'+11:00';
timezone

---------------------

2017-11-30 13:00:00

select '2017-12-01 11:00:00 -11:00'::timestamp with time zone at time zone
'+11:00';

timezone

---------------------

2017-12-01 11:00:00

The Table 8-12. Time Zone Input section at
https://www.postgresql.org/docs/9.3/static/datatype-datetime.html seems to
imply that numeric offsets would be treated as ISO8601. It's all a big
confusing and would appreciate some clarification or pointer to
documentation.

Thanks