Wrong Sorting done, when exported to pdf or excel

Here the number of columns when viewed on html media is different than when viewed under pdf or excel. Sorting is always done on the html columns. When the table gets exported to pdf or excel with more columns than what we see in html output, it sorts incorrectly based on the column numbers of html output.

 <display:table name="test" export="true" sort="list" pagesize="8">
    <display:column media="excel pdf" property="city" title="CITY" sortable="true" headerClass="sortable" />
    <display:column property="project" title="PROJECT" group="2" sortable="true" headerClass="sortable" />
    <display:column property="amount" title="HOURS" sortable="true"/>
    <display:column property="task" title="TASK" />
  </display:table>

In the above example,
In HTML output, I have 3 columns project, amount and task. If I click on amount ( column number is 1) sorting is done based on amount column
when we click export, the same column number goes to the Table Model to get the table sorted based on the same column number. But now in excel or pdf we have 4 columns and column number 1 represents to project. So now the table is sorted based on the project column title. This is a bug.