How are Deloitte hike percentages gets decided? I have seen couple of my friends who got very good rating but their hike is in single digit

Hike is decided by multiple factors. Like budget allocated to the team, billing %/ sakes revenue from the project, management view of criticality of the project, and future billing/revenue prospects. Ratings are merely a tool for the HR and managers to remind that they control "your" ability to be employed.

I have seen people get poor rating and better hike. I have seen people who fight with their managers and get a better hike when they resign. Ratings have the same purpose as umbrella on a windy day.