Why didn't this statistics object on a filtered index auto update?

In a SQL Server 2012 SP1 instance, I have a filtered index on the PK column of a table that looks like this:

CREATE INDEX [RF_IXF_Orders_OrderNumber] ON [dbo].[Orders]
(
    [OrderNumber] ASC
)
WHERE [OrderSource]='MO'
AND [Cancelled]=(0)
AND [NumItems]>(0)
AND [OrderDate]>'2014-05-15';

The Row Count (dm_db_partition_stats.row_count) of the index is 8416, the Rows Changed (sysindexes.rowmodctr) is 16803 (193.6%), and Auto Update Statistics is True. There have been 8400 user_scans and 50,088 user_updates. The last user_scan was today, yet the statistics haven't been updated in about a week.

Why wouldn't the statistics have auto updated?