When selecting a host in an automated DRS cluster, it is the vCenter server that makes the decision which ESX host in the cluster to use; if it is a manual DRS or non-DRS cluster, you can select the specific host. The user visible difference is that in the latter case there is an additional drop-down box for host selection.
Actually, this is a kind of user experience bug, because that difference in behavior is not clearly stated. The guide mentions it on p.49, but it is not clear cut either.
Regards,
Plamen