While there's the creative side, arts training also
values a
high degree of technical
aptitude, including an extreme attention to detail.
So if the statistical controls used did not effectively «control for» the lack of non random assignment of students into classrooms, teachers may have been assigned
high value - added scores not necessarily because they were
high value - added teachers but because they were non-randomly assigned
higher performing,
higher aptitude, etc. students, in the first place and as a whole.