Never really noticed - Although perhaps something in the web browsers that do it.
Although I am not a fan of Safari's UI, it does have colour correction so that colours look the same on every screen (or as near the same as possible) but not sure if that would effect brightness.
I recommened looking at images in a few different browsers side by side to see if they make any difference to the brightness etc.
That or the image editing application you are using dims them down for some reason - but obviously its not just to make them darker - I've noticed lightroom do this, loads the image, and from the image as it says loading at the bottom, it changes settings, but the sliders don't actually move, leading me to believe that it is some type of correction process for certain image formats.
Canon EOS 5DmkII + BG-E6 + Canon EOS 40D + BG-E2N + Canon EOS 33
Canon 17-40 F/4L
USM + Canon 24-70 F/2.8L
USM + Canon 28mm F/1.8 USM + Canon 70-200 F/2.8L
USM IS + Canon 85mm F/1.8 USM
Canon Speedlite 580EX II + Canon Speedlite 540EZ + 2 x Nikon SB-80DX
Cactus V2s Wireless Trigger - 5 x
Cactus V2s Wireless Reciever