Collaborative environment is a key to the Scrum success. Product owner, Scrum Master and Development Team should work together and there should be a mutual respect. Of course it takes time if you and or your team are new to Scrum.
Here are my thoughts -
"Developers tend to give higher estimates while creating tasks for user stories. We know developers updates the hours such that all estimated time consumed even if it takes less time and they spend time on the tasks other than task relevant to User Stories"
1. You are making generalized statement about the developers, which is not true in most scenarios. Also you have perception about the developer task estimation that they always quote more hours. The perception should change. If we want others to change, the change should come from us first.
2. Even though the Scrum Master can’t tell who should work on what and how it should be done, (Since Development Team is self-organized and cross functional) but he can guide the team if the task is irrelevant to the user story. Scrum Master can point out if the task is not targeted towards the acceptance criteria (AC) defined for the user story. All the tasks should be relevant to the AC. If the developer is working on a task completely irrelevant to the Sprint Goal or AC of a user story, then it is Scrum Master’s responsibility to re-iterate the same and make sure the team focuses on the AC and Sprint Goal.
3. If you feel that the development team is not committing enough (like story/task estimations are always high, you should try discuss this point during Sprint Retrospective meeting on how to improve team’s Velocity. Teach the team on Sprint estimation, Story sizing and relative size estimation, etc., May be development team has environment challenges so the estimates go high. Consider Xtreme programming techniques like Continuous integration, TDD, pair programming etc., Have your lead developer or Architect be a mentor to the developers and help them perform better.
4. If the development team is spending 10% of their time towards product backlog grooming, they should have better understanding over the story/task estimation during sprint planning.
Inspect & Adapt