"the headers of the columns with the 'unsortable' attribute are sized just like a sortable column, without the sorting button."
This could easily be improved, if we change the recently added
.client-js table.sortable th { padding-right: 21px; }
into
.client-js table.sortable th:not(.unsortable) { padding-right: 21px; }
Example table: http://reading-web-staging.wmflabs.org/wiki/Help:Sorting#Making_a_column_unsortable