HP 48GX screen contrast

[groskwik]

I often go to this forum but I realize this is my very first post !

The HP-48 GX is still for me one of the best calculator ever made. I've just bought another one for almost nothing in an amazing condition. So I have two GX. One is a 1996 revison R and one is a 1993 revision P (the "new" one).

I immediately saw two differences between these units :
- The screen of the rev. P as a better contrast.
- The color of the rev. P is more matte than the rev. R.

What surprises me more is the screen contrast. Why is it better on the older one ?
In the attached picture you can see the difference (even if it is easiest to see in real). If I press [On] [-] on the Rev R to lighten the background at the same level as the rev. P, the letters start to "disappear".