confused by the innodb gap lock

[Luke Lu]

i have a table like
create table t
(
a int primary key,
b int
) engine=innodb;

create index inx_t_b on t(b);
insert into t select 1,10;
insert into t select 2,10;
insert into t select 3,20;
insert into t select 4,20;
insert into t select 5,30;
insert into t select 6,30;
commit;
=============================
then in session A: isolation level: repeatable read
select * from t where b<=10 for update;
according to the innodb gap lock,this will lock the gap （-∞，10] and (10,20] on index inx_t_b
and lock the record 1 and 2 on primary key

in session B:isolation level: repeatable read
insert into t values (100,100)

It's observed the SQL statement in session B is blocked and finnaly it will be timeout and failed,because
it can't obtain the lock

So i am confused why the SQL in session A block SQL in session B?
what's the exact lock range for SQL in session A?

will some one help me out

the MySQL version is 5.6