All indexes are ordered. A cluster index is the data meaning that the data is ordered according to
the clustered index. However, "ordered" doesn't mean it is ordered physically. Pages splits etc lead
to fragmentation meaning that SQL Server will "jump around" in the data file while following the
linked list (order of the index). Because of this, it can be more beneficial to use the IAM page to
scan the file (relevant pages) physically instead of "jump around" in the database file. There are
good sections in Books Online, for instance the "Physical Database Architecture".

To add on to Tibor's response, Ordered=True tells the storage engine that
data must be returned in key sequence.  The ordered requirement is needed by
some execution plan operators (e.g. merge join) or to address ordering of
the result set (ORDER BY).

Ordered=False indicates that there is no requirement that data be returned
in a particular order so the storage engine might use scan techniques that
do not guarantee ordering, such as IAM or advanced scanning.