It looks like the Release Burndown only considers a Story's original estimate value, not the sum of the estimates for all the sub-tasks. Setting the Story's original estimate value to be the same as the sum of the estimates from it's sub-tasks will work for the Release Burndown but will screw up/skew the "Sum of Original Estimates" value on the Story (as it will add the Story Original Estimate to the rollup from all the Sub-tasks).
Is there a way to base the Release Burndown on the sum of the sub-task estimates?