Redmine

time tracking works weird with subtasks.
I logged to demo.redmine.org and created 3 tasks in my "ace" project:

1) feature1 - no estimate - target release 6.2

subtasks for feature1:
2) bug1 - estimate 5 hours - target release 6.2
3) task1 - estimate 10 hours - target release 6.2

I see total estimated time on 6.2 roadmap is 30 hours.
feature1 consists of 2 tasks, which are 15 hours total. and I see the total time of 15 hours shown on "feature1" details page.
why is subtasks' time added to the main task's time itself on the roadmap?

I'm considering using Redmine for simple project planning and I'm curious if timetracking feature is something experimental or stable enough to be used in production.