When a SCM poll is setup Jenkins polls a repository for changes.
When an external is referencing a specific revision (instead of head) the poll continues to check the base repository of that external for changes. If the revision of the original repository (containing the external) revision does not match the externals base head revision then it detects that there is an updated needed, this then triggers a build.
Subversion does an update but will not update the external to the revision that was detected because the external is pointing to a specific revision not the head. This then causes the poll job to continuously 'succeed' in getting updated revisions until the repositories external revision matches that of its base repositories head revision.
This is actually causing a lot of issues with Disk usage on our servers (it is a virtual server) and slowing other systems down.