Why the lp_len is 28 not 32?

[jack...@gmail.com]

use these sqls below:
create table t(a int);
insert into t values(1);
select lp,lp_off,lp_len,t_data from heap_page_items(get_raw_page('t',0));
 lp | lp_off | lp_len |   t_data   
----+--------+--------+------------
  1 |   8160 |     28 | \x01000000
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jack...@gmail.com