Online index rebuild higher fragmentation on intermediate level
Try this:
ALTER INDEX pk_test ON dbo.test REBUILD WITH (PAD_INDEX = OFF, 
    STATISTICS_NORECOMPUTE = OFF, 
    SORT_IN_TEMPDB = ON,  
    ONLINE = ON, 
    ALLOW_ROW_LOCKS = OFF, 
    ALLOW_PAGE_LOCKS = OFF, 
    FILLFACTOR = 90, 
    OPTIMIZE_FOR_SEQUENTIAL_KEY = ON)

This way you will have a FILLFACTOR of 90% and you would probably avoid to have the table fragmentation to spike up again.
Keep in mind that your table size will increase of 10%