imac A1312 brightness issue

This are 2 different imac but Ive also validated whitin the same pc.

I notice a max brightness dicrease i switch from booting oclp from the ssd or booting from the usb.

Im pretty new to this, but does it even makes sense?

Anyone have gone through this? Or am I missing something?

I have both pcs on right now at max bright and notice a big difference. Also both displays are ok.