One-off failure in "cluster" test

Hi,

I wonder what caused this[1] one-off failure to see tuples in clustered order:

diff -U3 /home/pgbfarm/buildroot/REL_13_STABLE/pgsql.build/src/test/regress/expected/cluster.out
/home/pgbfarm/buildroot/REL_13_STABLE/pgsql.build/src/test/regress/results/cluster.out
--- /home/pgbfarm/buildroot/REL_13_STABLE/pgsql.build/src/test/regress/expected/cluster.out
2020-06-11 07:58:23.738084255 +0300
+++ /home/pgbfarm/buildroot/REL_13_STABLE/pgsql.build/src/test/regress/results/cluster.out
2020-07-05 02:35:06.396023210 +0300
@@ -462,7 +462,8 @@
where row(hundred, thousand, tenthous) <= row(lhundred, lthousand, ltenthous);
hundred | lhundred | thousand | lthousand | tenthous | ltenthous
---------+----------+----------+-----------+----------+-----------
-(0 rows)
+ 0 | 99 | 0 | 999 | 0 | 9999
+(1 row)

I guess a synchronised scan could cause that, but I wouldn't expect one here.

[1] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=chipmunk&dt=2020-07-04%2023:10:22