Unexpected Backend PID reported by Notification

Hi. I have a unit test using a single connection, that simulates a
client interacting with a server via a PostgreSQL "queue", i.e. a
non-writable table with SECURITY DEFINER procedures to mediate writes
to that table, with those PROC-initiated updates triggering
pg_notify() messages (via an UPDATE trigger).

The test is passing, I get all the side-effects and notifications I
expect. BUT...
For some reason, the backend_pid reported on the notification object
itself (i.e. PGnotify::be_pid),
is different from the one reported for the (sole) connection the unit
test is using (PQbackendPID()).

How can that be?
Are Stored PROCs running in a different backend?
Are Triggers running in a different backend?

Any doc pointers to explain this behavior?

Thanks. --DD

PS: v14 server on RedHat; v16 libpq on Windows
PPS: Below's a snippet of my test code, which shows actual PID values:

    auto perreq_notif = c.notification();
    BOOST_REQUIRE(perreq_notif);
    BOOST_CHECK_EQUAL(perreq_notif.channel(), req.channel());
    /*
    ** In fact I get perreq_notif.backend_pid() == N + c.backend_pid() !!!
    ** Is the fact the pg_notify() is done from a trigger the reason???
    ** e.g. [4053957 != 4053955]
    BOOST_CHECK_EQUAL(perreq_notif.backend_pid(), c.backend_pid());
    */
    BOOST_CHECK_EQUAL(perreq_notif.payload(), "...");