Changing explicit_defaults_for_timestamp within stored procedure works inconsistently

--delimiter $

create procedure pr1()

begin

  set explicit_defaults_for_timestamp= off;

  create table t1 (ts timestamp);

end $

create procedure pr2()

begin

  set explicit_defaults_for_timestamp= on;

  create table t2 (ts timestamp);

end $

--delimiter ;

set explicit_defaults_for_timestamp= on;

call pr1();

select @@explicit_defaults_for_timestamp;

show create table t1;

set explicit_defaults_for_timestamp= off;

call pr2();

select @@explicit_defaults_for_timestamp;

show create table t2;

# Cleanup

drop procedure pr1;

drop procedure pr2;

drop table t1, t2;

set explicit_defaults_for_timestamp= on;

call pr1();

select @@explicit_defaults_for_timestamp;

@@explicit_defaults_for_timestamp

0

show create table t1;

Table	Create Table

t1	CREATE TABLE `t1` (

  `ts` timestamp NULL DEFAULT NULL

) ENGINE=MyISAM DEFAULT CHARSET=latin1

set explicit_defaults_for_timestamp= off;

call pr2();

select @@explicit_defaults_for_timestamp;

@@explicit_defaults_for_timestamp

1

show create table t2;

Table	Create Table

t2	CREATE TABLE `t2` (

  `ts` timestamp NOT NULL

) ENGINE=MyISAM DEFAULT CHARSET=latin1