Maybe so, that's what i thought initially,
until I looked at how I was grabbing it, etc.
It's very easy to accidently turn the wheel
without even realizing, just setting it down for example, and then as a result having to readjust
contrast before re-use.

Because like you i wasn't sure whether it was
screen output variations, I put a little clear tape over the wheel and ran it like that for
a few days, and the result was no discernable
contrast variations (that you couldn't attribute
to room lighting).

Oh well, I got very little to complain about
with the Pronto anyways. Maybe it has more to
do with my 'Bratwurst' style fingers than
Philips choice of controls.