The time taken might be factored in but:

* On busy streets, left turns through uncontrolled intersections can have high variability. So you might be unlucky and get stuck for several minutes waiting to turn. A slower, but more predictable route might be better if you need to arrive on time.

* Left turns are simply more stressful than right turns. Saving a minute might not be worth the pain.

Your variability point extends beyond just left turns. Routes with lots of lights or that cross train tracks have extra variability. If I'm leaving with barely enough time I want directions that have low time variance since I cannot afford to be on the wrong side of the distribution. Other times I'm willing to risk some variance for a shorter expected time.