The other (possibly controversial) thing worth pointing out is that no amount of estimation, planning or project management will change the actual time taken to implement a feature. This is absolute, and dependent solely on the ability of the developer responsible for doing the work. The PM's role in an agile team should be to ensure that developers know what they should be doing, and then to remove anything that prevents them from doing it. In a productive team the PM is a supporting role, not the primary role.

The high-level estimate will never match the task-level estimate - it's just not possible to understand how a feature will work until you get into the detail. So my advice is not to bother trying to correlate them. The feature / story level estimate is mainly useful as a guide to the PO for him/her to understand whether something is difficult / hard to achieve. It's relative - "this feature is harder than that feature" etc.