Earlier quoted context omitted.
6.4/5.2 = 1.23 . Is 23% improvement considered that small(assuming it's statistically significant) ?
I think you're looking at the wrong derivative. This is 23% improvement of improvement. It's still just 6.4% improvement. Might be easier to see if you think of it as 106.6/105.2, in order to include the baseline.
I'm not sure 106.6/105.2 is the correct way to look at it either.
Intuitively, every point you got in a test corresponds to a micro-subject.
So before the class , you knew 100 micro-subjects(which probably took a few years to teach). And you gained 6.4 or 5.2 micro-subjects during the 5 week class.
Since one class taught 6.4 micro-subjects and the other only 5.2 micro-subjects ,you can say the difference is 32%.