ALTER INDEX (with REBUILD), if run several times both decreases and increases the average fragmentation for an index

ALTER INDEX (with REBUILD), if run several times both decreases and increases the average fragmentation for the non-clustered index.  I would expect the average fragmentation to be 0 after the first run of ALTER INDEX.
 
Below is the sequence of events and results: 
Running the following SQL:

SELECT a.index_id, b.name, a.avg_fragmentation_in_percent FROM sys.dm_db_index_physical_stats ( DB_ID(N'tmac'), OBJECT_ID(N'dbo.WS_Schedule_cur'), NULL, NULL, NULL ) AS a JOIN sys.indexes AS b ON a.object_id = b.object_id AND a.index_id = b.index_id;  

produced: 

index_id   name                    avg_fragmentation_in_percent
1          PK_WS_Schedule_cur      14.0845070422535
15         WS_Schedule_cur_idx1    94.488188976378

I then ran SQL:

ALTER INDEX ALL ON dbo.WS_Schedule_cur REBUILD WITH (FILLFACTOR = 100, SORT_IN_TEMPDB = ON, STATISTICS_NORECOMPUTE = ON);

I then ran the ALTER INDEX, and the 1st SQL which produced:

index_id   name                    avg_fragmentation_in_percent
1          PK_WS_Schedule_cur      0
15         WS_Schedule_cur_idx1    42.8571428571429

Going from 14% to 0% for the PK is what I expected for both indexes. 
I am satisfied with going from 94% to 42% for the non-clustered index,
but why did I not get 0%?

Subsequent runs of the ALTER INDEX, and 1st SQL produced the following
percentages for the non-clustered index
which sometimes decreases and sometimes increases.  Why?

42.8571428571429
42.8571428571429
28.5714285714286
28.5714285714286 
21.4285714285714
21.4285714285714
35.7142857142857
28.5714285714286

Here is the DDL for the table, PK, and non-clustered index:

CREATE TABLE [dbo].[WS_Schedule_cur]( [Schedule_Id] [int] NOT NULL, [Schedule_Name] [varchar](30) NOT NULL, [Schedule_Owner] [varchar](4) NOT NULL, [Source_CA] [varchar](4) NOT NULL, [Sink_CA] [varchar](4) NOT NULL, [PSE] [varchar](6) NULL, [Product_Code] [tinyint] NULL, [Source] [varchar](30) NULL, [Sink] [varchar](30) NULL, [Flow] [char](8) NULL, [Schedule_Type] [varchar](20) NOT NULL, [Schedule_SubType] [int] NULL, [Schedule_Status] [varchar](12) NOT NULL, [Schedule_Start] [datetime] NOT NULL, [Schedule_End] [datetime] NOT NULL, [Time_Of_Last_Update] [datetime] NOT NULL, CONSTRAINT [PK_WS_Schedule_cur] PRIMARY KEY CLUSTERED ( [Schedule_Id] ASC)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = ON, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON, FILLFACTOR = 100) ON [PRIMARY]) ON [PRIMARY]CREATE NONCLUSTERED INDEX [WS_Schedule_cur_idx1] ON [dbo].[WS_Schedule_cur] ( [Schedule_Name] ASC)INCLUDE ( [Schedule_Id],[Schedule_Status]) WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = ON, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON, FILLFACTOR = 100) ON [PRIMARY]