Here I see the biggest problems with the studies. Lets say you have a 100 healthy hair and 100 vellus. Healthy hair diameter change will be 0 and vellus +20%. In total increase is 10%.That sounds actually quite good.
Now lets say you have 150 healthy hair and 50 vellus. Healthy +0 and vellus +40%. This is awesome result.
But it can be also exactly other way. Healthy hair change +20% and vellus 0. This does not sound as awesome....
Thats why writting that the change of hair diameter, but not of which type will always be kind of misleading.