Upon checking on the documentation on the columns for issue navigator, was not able to find any recent information above the usage of the Σ Original Estimate, Σ Remaining Estimate and Σ Time Spent. The only information that available back traces to the release note of Jira version 3.11 that is no longer supported.

• https://confluence.atlassian.com/display/JIRA/JIRA+3.11+Release+Notes#JIRA3.11ReleaseNotes-IssueNavigatorofferssub-taskaggregates

And according to the documentation:

Σ Original Estimate — the aggregate Original Estimate for an issue's sub-tasks.

But the actual test on the latest Jira version 5.2.6 confirms that the value of Σ Original Estimate column is different from the earlier claim. if you viewed the Original Estimate time for the Main Task, it actually includes the Original Estimate on the Main Task itself together with the Original Estimate defined on the sub tasks.

As an example:

I created a main task with an original estimate of 1 week, sub task 1 with original estimate of 1 day and sub task 2 with original estimate of 2 days. As you can see when you view the Σ Original Estimate of the Main Task, it shows 1 week 3 days instead of 3 days alone.