Why is the AggregateMultiTenancyIsolation filter not working as expected ?

Issue

When implementing the AggregateMultiTenancyIsolation filter from scratch, and having followed the instructions, the spawned instances are placed all across the cluster, even though the host aggregate is specified in the hosts

# nova aggregate-details test_ha
+----+-------------+-------------------+--------------------------------------------------------+-----------------------------------------------------+
| Id | Name        | Availability Zone | Hosts                                                  | Metadata                                            |
+----+-------------+-------------------+--------------------------------------------------------+-----------------------------------------------------+
| 23 | test_ha     | -                 | 'compute001.openstack.pxe', 'compute002.openstack.pxe' | 'filter_tenant_id=6e70632342474845b7b9273ef123456b' |
+----+-------------+-------------------+--------------------------------------------------------+-----------------------------------------------------+

Here, it can be seen that the Filter is returning more than 2 hosts:

2015-07-22 10:58:07.744 2529 DEBUG nova.filters [req-2409046f-e03d-463a-ac69-717cf0b7b2d7 None] Filter AggregateMultiTenancyIsolation returned 36 host(s) get_filtered_objects /usr/lib/python2.7/site-packages/nova/filters.py:88