Btree internal node data?

While looking into a btree internal page using pg_filedump against an
int4 index generated pgbench, I noticed that only item 2 has length 8,
which indicates that the index tuple has only tuple header and has no
index data. In my understanding this indicates that the item is used
to represent a down link to a page. Question is, why the item is 2,
not 1. I thought an index tuple indicating down link is always 1. Is
this a sign that something goes wrong?

Block    3 ********************************************************
<Header> -----Block Offset: 0x00006000         Offsets: Lower    1164 (0x048c)Block: Size 8192  Version    4
Upper   3624 (0x0e28)LSN:  logid      2 recoff 0x1550a608      Special  8176 (0x1ff0)Items:  285
FreeSpace: 2460Checksum: 0x0000  Prune XID: 0x00000000  Flags: 0x0000 ()Length (including item array): 1164
 

<Data> ------ Item   1 -- Length:   16  Offset: 3624 (0x0e28)  Flags: NORMALItem   2 -- Length:    8  Offset: 8168
(0x1fe8) Flags: NORMALItem   3 -- Length:   16  Offset: 8152 (0x1fd8)  Flags: NORMALItem   4 -- Length:   16  Offset:
8136(0x1fc8)  Flags: NORMALItem   5 -- Length:   16  Offset: 8120 (0x1fb8)  Flags: NORMAL
 
[snip]Item 281 -- Length:   16  Offset: 3704 (0x0e78)  Flags: NORMALItem 282 -- Length:   16  Offset: 3688 (0x0e68)
Flags:NORMALItem 283 -- Length:   16  Offset: 3672 (0x0e58)  Flags: NORMALItem 284 -- Length:   16  Offset: 3656
(0x0e48) Flags: NORMALItem 285 -- Length:   16  Offset: 3640 (0x0e38)  Flags: NORMAL
 

<Special Section> -----BTree Index Section: Flags: 0x0000 () Blocks: Previous (0)  Next (289)  Level (1)  CycleId (0)

Best regards,
--
Tatsuo Ishii
SRA OSS, Inc. Japan
English: http://www.sraoss.co.jp/index_en.php
Japanese:http://www.sraoss.co.jp