Use of nonclustered index
You confuse what with index  means.  Use of an index in no way directly drives the order of the results.  A table does not have an inherent order.  Even a table with a clustered index may not return results in the order of clustered index.  Now a sort will use an index but the index alone does not force the sort.  In your case the index made the sort efficient and I suspect if you look at the query plan the sort used the index.