Para a definição da tabela

CREATE  TABLE Accounts
(
AccountID INT ,
Filler CHAR(1000)
)

Contendo 21 linhas (7 para cada um dos valores de AccountId4,6,7).

Tem 1 página raiz e 4 páginas folha

index_depth page_count           index_level
----------- -------------------- -----------
2           4                    0
2           1                    1

A página raiz se parece com

FileId      PageId      ROW         LEVEL       ChildFieldId ChildPageId AccountId (KEY) UNIQUIFIER (KEY) KeyHashValue
----------- ----------- ----------- ----------- ------------ ----------- --------------- ---------------- ------------------------------
1           121         0           1           1            119         NULL            NULL             NULL
1           121         1           1           1            151         6               0                NULL
1           121         2           1           1            175         6               3                NULL
1           121         3           1           1            215         7               1                NULL

A distribuição real deAccountId registros nessas páginas é

AccountID   page_id     Num
----------- ----------- -----------
4           119         7
6           151         3
6           175         4
7           175         1
7           215         6

A pergunt

SELECT AccountID 
FROM Accounts 
WHERE AccountID IN (4,6,7) 

Dá as seguintes estatísticas de IO

Table 'Accounts'. Scan count 3, logical reads 13

Por quê

Pensei que, para cada busca, procuraria na primeira página que pudesse conter esse valor e, em seguida (se necessário) continue ao longo da lista vinculada até encontrar a primeira linha diferente do valor procurad

No entanto, isso adiciona apenas até 10 acessos à página

4)  Root Page -> Page 119 -> Page 151             (Page 151 Contains a 6 so should stop)
6)  Root Page -> Page 119 -> Page 151 -> Page 175 (Page 175 Contains a 7 so should stop)
7)  Root Page -> Page 175 -> Page 215             (No more pages)      

Então, o que representa os 3 adicionais?

Script completo para reproduzir

USE tempdb

SET NOCOUNT ON;

CREATE  TABLE Accounts
(
AccountID INT ,
Filler CHAR(1000)
)

CREATE CLUSTERED INDEX ix ON Accounts(AccountID)


INSERT INTO Accounts(AccountID)
SELECT C
FROM (SELECT 4 UNION ALL SELECT 6 UNION ALL SELECT 7) Vals(C)
CROSS JOIN (SELECT TOP (7) 1 FROM master..spt_values) T(X)

DECLARE @AccountID INT

SET STATISTICS IO ON
SELECT @AccountID=AccountID FROM Accounts WHERE AccountID IN (4,6,7) 
SET STATISTICS IO OFF

SELECT index_depth,page_count,index_level
FROM
sys.dm_db_index_physical_stats (2,OBJECT_ID('Accounts'), DEFAULT,DEFAULT, 'DETAILED')

SELECT AccountID, P.page_id, COUNT(*) AS Num
FROM Accounts
CROSS APPLY sys.fn_PhysLocCracker(%%physloc%%) P
GROUP BY AccountID, P.page_id
ORDER BY AccountID, P.page_id

DECLARE @index_info  TABLE
(PageFID  VARCHAR(10), 
  Pa,gePID VARCHAR(10),   
  IAMFID   TINYINT, 
  IAMPID  INT, 
  ObjectID  INT,
  IndexID  TINYINT,
  PartitionNumber TINYINT,
  PartitionID BIGINT,
  iam_chain_type  VARCHAR(30),    
  PageType  TINYINT, 
  IndexLevel  TINYINT,
  NextPageFID  TINYINT,
  NextPagePID  INT,
  PrevPageFID  TINYINT,
  PrevPagePID INT, 
  PRIMARY KEY (PageFID, PagePID));

INSERT INTO @index_info 
    EXEC ('DBCC IND ( tempdb, Accounts, -1)'  ); 

DECLARE @DynSQL NVARCHAR(MAX) = 'DBCC TRACEON (3604);'
SELECT @DynSQL = @DynSQL + '
DBCC PAGE(tempdb, ' + PageFID + ', ' + PagePID + ', 3); '
FROM @index_info     
WHERE IndexLevel = 1

SET @DynSQL = @DynSQL + '
DBCC TRACEOFF(3604); '

CREATE TABLE #index_l1_info  
(FileId  INT, 
  PageId INT,   
  ROW   INT, 
  LEVEL  INT, 
  ChildFieldId  INT,
  ChildPageId INT,
  [AccountId (KEY)] INT,
  [UNIQUIFIER (KEY)] INT,
  KeyHashValue  VARCHAR(30));

INSERT INTO #index_l1_info  
EXEC(@DynSQL)


SELECT *
FROM #index_l1_info

DROP TABLE #index_l1_info
DROP TABLE Accounts