Any brand of "bright" screens shows you a "bright" image because they are so designed as to concentrate most of the light coming from the groundglass toward the eyecup... and your eye.
But the photocells are not there, they are beside the eyecup ; so, the screen may not be so bright for them, and there is no evident relationship between the brightness that you view and the brightness that your photocells see. Maybe even that this relationship depends on the lens aperture at which you measure the light with the photocell... "Sursum corda !" and use a separate lightmeter.
Paul