Very good Question. I had raised same long back. I even asked all my fellow sellers had it ever happened that their feedback was removed other than 51% majority. But I found not a single feedback had been removed with any other majority but 51%.
I tried finding reason. Got one unofficially.
Jury starts voting, as soon as 11 votes are caste in favor of removal, voting closes and feedback removal is approved, (wait is not done for 21 votes as, 11 needed for majority are already obtained).
Therefore majority of 51% vote always comes.