SAT is not computer-graded, but by 2 humans, apparently.
The GRE is graded by a human and a computer, and a second human in case of diverging scores.
SAT: Responses to the optional SAT Essay are scored using a carefully designed process.
Two different people will read and score your essay.
Each scorer awards 1–4 points for each dimension: reading, analysis, and writing.
The two scores for each dimension are added.
You’ll receive three scores for the SAT Essay—one for each dimension—ranging from 2–8 points.
There is no composite SAT Essay score (the three scores are not added together) and there are no percentiles.
GRE: For the Analytical Writing section, each essay receives a score from at least one trained rater, using a six-point holistic scale. In holistic scoring, raters are trained to assign scores on the basis of the overall quality of an essay in response to the assigned task. The essay is then scored by e-rater®, a computerized program developed by ETS that is capable of identifying essay features related to writing proficiency. If the human and the e-rater scores closely agree, the average of the two scores is used as the final score. If they disagree, a second human score is obtained, and the final score is the average of the two human scores.
