Datatable Plug-Ins with Rails

I'm trying to use the sorting by 'Hidden title numeric sorting' but this is not sorting by the title's value but the actually displayed value. What am I doing wrong?

This is in the application.js:
//= require dataTables/js/jquery.dataTables
//= require datatables/extras/sorting/title-numeric

This is the table-cells:
<td><span title='2'>20/192</span></td>
<td><span title='3'>30/192</span></td>
<td><span title='1'>10/192</span></td>