BUG #14381: justify_interval function works in strange way

The following bug has been logged on the website:

Bug reference: 14381
Logged by: Maciej Srebrny
Email address: maciej(dot)srebrny+postgresql(at)gmail(dot)com
PostgreSQL version: 9.3.14
Operating system: AWS - RDS
Description:

I already read that justify_interval function have some limits (BUG-14313)
but this behaviour is very not intuitive:

select justify_interval(current_timestamp - current_timestamp + interval '8
year');
justify_interval
------------------
8 years

select justify_interval(current_timestamp - (current_timestamp - interval '8
year'));
justify_interval
-----------------------
8 years 1 mon 12 days