EXPLAIN SELECT returns inconsistent number of ROWS in main.group_by

Copy of https://bugs.mysql.com/bug.php?id=93703:

Description:
EXPLAIN SELECT sometimes returns 4 or 5 as "rows" what is reported as errors for `main.group_by`:

— /tmp/results/PS/mysql-test/r/group_by.result 2018-12-19 13:10:55.000000000 +0300
+++ /tmp/results/PS/mysql-test/var/4/log/group_by.reject 2018-12-19 15:29:24.416116648 +0300
@@ -1752,7 +1752,7 @@
Note 1003 /* select#1 */ select `test`.`t2`.`a` AS `a`,sum(`test`.`t2`.`b`) AS `SUM(b)` from `test`.`t2` group by `test`.`t2`.`a` limit 2
EXPLAIN SELECT a, SUM(b) FROM t2 IGNORE INDEX (a) GROUP BY a LIMIT 2;
id select_type table partitions type possible_keys key key_len ref rows filtered Extra
-1 SIMPLE t2 NULL ALL a NULL NULL NULL 5 100.00 Using temporary
+1 SIMPLE t2 NULL ALL a NULL NULL NULL 4 100.00 Using temporary

How to repeat:
The minimal MTR test to show the issue:

CREATE TABLE t1 (a INT, b INT, KEY(a));
INSERT INTO t1 VALUES (1, 1), (2, 2), (3,3), (4,4);

CREATE TABLE t2 (a INT, b INT, KEY(a));
INSERT INTO t2 VALUES (1, 1), (2, 2), (3,3), (4,4);

EXPLAIN SELECT a FROM t1 IGNORE INDEX (a) GROUP BY a;
EXPLAIN SELECT a FROM t2 IGNORE INDEX (a) GROUP BY a;

EXPLAIN SELECT a FROM t1 GROUP BY a;
EXPLAIN SELECT a FROM t2 GROUP BY a;