It's been perfectly normal for some years now. My 3 year-old dumb SE does it too
It uses a light sensor on the face of the device, close to the front camera (check the manual for the exact location). Covering it (with your finger or something) will keep the screen at it's dimmest.
I have a feeling it's strictly hardware, so there is no way to change the way it works. Could be different on the n900 though