[dba-SQLServer] Index Fragmentation

Now I am very puzzled. After noticing a number of seriously fragmented
indexes, I ran a job over the weekend, using the following syntax:

ALTER INDEX IX_900_PortfolioSummary__PPCID_PortID ON
dbo.[900_PortfolioSummary] REBUILD
etc.

To my surprise, the indexes in question remain just as fragmented as they
were before the job. Can anyone advise me why this is so? I thought that a
REBUILD completely rebuilt the index with no fragmentation.

Thanks,
Arthur