Rating drivers out of 10 is an inexact and frustrating science. Fascinating as it is to track each driver's progress, to boil down their performance over the weekend to a number is impossible to do completely objectively.
The ratings reflect various sources of information - laptimes, feedback from teams, performance in battle, mistakes, pace relative to team-mate - but there is inevitably some margin for error.
Still, a glance at the average ratings at the end of the season is always an instructive exercise as part of a review of each driver's performance.