Why does the "Total estimate" or "Total remaining" field not match the sum of the tasks?

If you have a backlog item with tasks, and the "Total estimate" or "Total remaining" field does not match the sum of the "Estimate" or "Remaining" fields of the tasks, it's because the task estimation options of the backlog item are configured to not use the sums.

You can easily change that in the task estimation options for the backlog item. You may also want to change the default task estimation options for the project. Note that changing the task estimation options for the project doesn't affect existing backlog items that already have tasks, so you need to change the task estimation options individually in each of those existing backlog items, too.