Let's then assume that 2 different thumb tests have been developed over time. I suppose then, that you can choose which one you prefer to use and which one makes the most sense to you. As long as it does make sense and you know why you are using it, yes?
On a side note:
Finger sensitivity was brought up by Joe. I don't have my old neurology books handy, but I do remember that the human finger tip is wired to be extremely sensitive. With training, one can descriminate very fine detail with light touch.
Here's a neat trick that you can try , if you really care. I suspect most won't care a rats @$$, but for those that do:
Take a phone book and place a hair under one page. Lightly brush your finger tips over the page and see if you can detect the hair. Now try it with 2 pages over the hair. Keep going until you can no longer find the hair. How many pages did it take? With practice, you might be amazed at how many pages you can cover the hair with and still feel it. A variation is to have someone else place the hair so you don't know exactly where it is, and see if you can find it with light touch.
Inquiring minds want to know...
Scott