0 is not distinct from 0.0 since (0 IS 0.0) IS TRUE.  So the query

<blockquote><verbatim>
SELECT DISTINCT * FROM (VALUES(0),(0.0));
</verbatim></blockquote>

Is free to return either 0 or 0.0.  The choice is arbitrary.  It is analogous
to a SELECT with a "LIMIT 1" and without an ORDER BY clause in that the query
is free to return any single row that it wants.

If the query decides to return 0, then 0||0.1 is a string '00.1' which
evaluates to TRUE.  If the query decides to return 0.0, then 0.0||0.1 is 
a string '0.00.1' which evaluates to false.  So the other queries in the
original problem statement are free to return either result depending on
which value comes out of the ambiguous view.