Official builds are tested with mysql-test-run --sleep=10, and the slave

has (in mysql-test-run) a slave-net-timeout=10. So the 'sleep 1' below
was converted in 'sleep 10', so slave thought that connection was down,
so reconnected, so got an additional (fake) Rotate event, which shifted
the positions displayed by SHOW SLAVE STATUS.

mysql-test/t/rpl_rotate_logs.test

@@ -101,7 +101,8 @@ show master logs;
 # we just tests if synonyms are accepted
 purge binary logs to 'master-bin.000002';
 show binary logs;
---sleep 1;
+# sleeping 10 seconds or more would make the slave believe connection is down
+--real_sleep 1;
 purge master logs before now();
 show binary logs;
 insert into t2 values (65);