How can an identity primary key index become fragmented?
Updates on data already there causes rows to be moved and forward pointers added

In this test, we can get 65% fragmentation on 115k densely packed rows

CREATE TABLE #FragTest (
    FragTestID int NOT NULL IDENTITY PRIMARY KEY,
    SomeString varchar(4100) NULL
    );

INSERT #FragTest (SomeString) VALUES ('a');
GO
INSERT #FragTest (SomeString)
    SELECT F1.SomeString FROM #FragTest F1 CROSS JOIN #FragTest F2;
GO 4
INSERT #FragTest (SomeString) SELECT F1.SomeString FROM #FragTest F1    
GO 6

SELECT COUNT(*) FROM #FragTest

SELECT object_id, avg_fragmentation_in_percent, page_count
FROM sys.dm_db_index_physical_stats(2, OBJECT_ID('tempdb..#FragTest'), NULL, NULL, NULL);

UPDATE #FragTest
SET SomeString = REPLICATE('b', 4100)
WHERE FragTestID &lt; 10000 AND FragTestID % 3 = 0

SELECT object_id, avg_fragmentation_in_percent, page_count
FROM sys.dm_db_index_physical_stats(2, OBJECT_ID('tempdb..#FragTest'), NULL, NULL, NULL);

UPDATE #FragTest
SET SomeString = REPLICATE('c', 4100)
WHERE FragTestID &lt; 10000 AND FragTestID % 3 = 1

SELECT object_id, avg_fragmentation_in_percent, page_count
FROM sys.dm_db_index_physical_stats(2, OBJECT_ID('tempdb..#FragTest'), NULL, NULL, NULL);

UPDATE #FragTest
SET SomeString = REPLICATE('d', 4100)
WHERE FragTestID &lt; 10000 AND FragTestID % 3 = 2

SELECT object_id, avg_fragmentation_in_percent, page_count
FROM sys.dm_db_index_physical_stats(2, OBJECT_ID('tempdb..#FragTest'), NULL, NULL, NULL);


Edit, sorry, browser trouble

If you change the middle replicate to 2000 you get &lt;5% at the end. This happens because there is free space from moved rows in the first update