Possibility 1: They're tiny tables or heaps. Small tables you don't need to worry about, heaps need a different command to rebuild (and besides, fragmentation means a different thing for heaps as opposed to indexes)
Possibility 2: You have a manual or automatic shrink running after the index rebuild.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability