It's easy to evaluate a Teacher by those metrics but that does not guarantee that the content the teacher is teaching is correct or accurate. Thus my point above.
Fractions and decimals are not difficult. I doubt that kids who have been learning math with Khan have problems understanding them. But ìf Khan has been a little unclear for some percentage of students, the addition of a few exercises should solve the problem. The lacunae of a smart teacher are easily fixed, unlike the confusion caused by a teacher who doesn't understand the material.
You must realize that this is an incredibly simplistic view of learning? How students learn is an extremely complicated process. While decimals are not difficult, understanding exactly where students struggle with the concept and determining a sequence of learning activities to help them overcome that concept is extremely difficult.