The grade of the route displayed is the median of the opinions from all the people who have ticked it.
The default option when ticking a route is to agree with the current grade. Another option would be to "force" people to choose a grade or give them with "no opinion" option.
Reasons why "current grade" is the default choice
1. The goal is to make ticking routes as easy as possible, and that's why the current grade is chosen by default. So users don't need to click many times to tick a route if they agree with the current grade. Also, it's still easy to give your opinion if you disagree!
2. The reason why "no opinion" is not the default option, is because then very likely only those climbers who disagree with the grade would give their opinion. This would lead to an inaccurate median and easily changing grades.