Type of text literals

Can someone please help me understand this:

b2bc_dev=# select pg_typeof('a') as a, pg_typeof(array['a']) as arr;
    a    |  arr
---------+--------
 unknown | text[]
(1 row)

Why is a text literal on its own 'unknown' whereas in the context of an
array it assumes 'text' ?

This is pg 10.4 in case it matters.

Thanks,

Steve