Inserts, updates, and deletes spoil indexes which start out good after create index or reindex.

Use the following DBCC command to check the state of an index:

dbcc showcontig(TableX) with all_indexes

Results:

LEAF level scan performed.
- Pages Scanned................................: 22329
- Extents Scanned..............................: 2804
- Extent Switches..............................: 2804
- Avg. Pages per Extent........................: 8.0
- Scan Density [Best Count:Actual Count].......: 99.54% [2792:2805]
- Logical Scan Fragmentation ..................: 12.56%
- Extent Scan Fragmentation ...................: 3.28%
- Avg. Bytes Free per Page.....................: 2146.7
- Avg. Page Density (full).....................: 89.77%

The fragmentation is 12.56%, in the tolerable range.