Index fragmentation problem after disabling page-level locking for an Index
The answer to your query is buried in some documentation:


  Online index rebuilding may increase fragmentation when it is allowed to run with MAX DOP > 1 and ALLOW_PAGE_LOCKS = OFF options.


The reasons for this are explained in detail in the following MSDN blog post:

How It Works: Online Index Rebuild – Can Cause Increased Fragmentation

Some options to resolve the problem in your situation, where page locks are disabled for the index, are:


rebuilding offline
rebuilding at MAXDOP 1