MERGE output doubt

[Luca Ferrari <fluca1978@xxxxxxxxx>]

Hi all,
I'm experimenting with the new MERGE command, but I'm not getting the
output total count. From the docs
<https://www.postgresql.org/docs/15/sql-merge.html>: "The total_count
is the total number of rows changed ".
This is my simple setup:

testdb=> table scores;
pk |   name   | score
----+----------+-------
 1 | luca     |    10
 2 | luca     |    20
 3 | luca     |    50
 4 | emanuela |    50
 5 | emanuela |   150
 6 | luca     |   122
(6 rows)

testdb=> truncate average_scores ;
TRUNCATE TABLE
testdb=> MERGE INTO average_scores a
USING ( SELECT name, avg( score ) AS avg_score
       FROM scores GROUP BY name ) s
ON s.name = a.name
WHEN MATCHED  THEN
   do nothing
WHEN NOT MATCHED THEN
   INSERT (name, avg_score) VALUES( s.name, s.avg_score );
MERGE 2

So far so good, two cumulative entries have been inserted into
average_scores. Now, if I use  a do nothing merge:

estdb=> MERGE INTO average_scores a
USING ( SELECT name, avg( score ) AS avg_score
       FROM scores GROUP BY name ) s
ON s.name = a.name
WHEN MATCHED  THEN
   do nothing
WHEN NOT MATCHED THEN
   do nothing;
MERGE 2

I was expecting an output tag like "MERGE 0" since both branches have
"do nothing", so no tuples should be updated at all on the target
table.
Moreover, if I truncate the target table and execute again the merge
query, I got the result of 0:

testdb=> truncate average_scores ;
TRUNCATE TABLE
testdb=> MERGE INTO average_scores a
USING ( SELECT name, avg( score ) AS avg_score
       FROM scores GROUP BY name ) s
ON s.name = a.name
WHEN MATCHED  THEN   -- caso di match
   do nothing
WHEN NOT MATCHED THEN
   do nothing;
MERGE 0

What am I missing here?