Running vacuum after delete does not remove all space allocated

I have deleted the rows in a table and vacuumed full, there appears to be space allocated that after a truncate it removes.

Why is this?

\d t8000_us_ts_size_test_2d
Table "dbprc001.t8000_us_ts_size_test_2d"
Column | Type | Modifiers
---------------+-----------------------+-----------
instrument_id | character varying(13) | not null
value | numeric(18,6)[] |
Indexes:
"t8000_us_ts_size_test_2d_pkey" PRIMARY KEY, btree (instrument_id)

pgdb001=# select pg_size_pretty(pg_total_relation_size('t8000_us_ts_size_test_2d'));
pg_size_pretty
----------------
13 MB

pgdb001=# delete from t8000_us_ts_size_test_2d;
DELETE 6

pgdb001=# vacuum full t8000_us_ts_size_test_2d;
VACUUM

pgdb001=# select pg_size_pretty(pg_total_relation_size('t8000_us_ts_size_test_2d'));
pg_size_pretty
----------------
12 MB

pgdb001=# truncate t8000_us_ts_size_test_2d;
TRUNCATE TABLE

pgdb001=# select pg_size_pretty(pg_total_relation_size('t8000_us_ts_size_test_2d'));
pg_size_pretty
----------------
16 kB

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