They have a different default gamma and they may show a different gray level.
(It bite me a long time ago. I made a gif that has the same RGB bacground than a webpage. In my pc it was fine, but in a mac they the border was very visible and the result horrible. My solution was to change the backgroung of the webpage from a RGB number to a 1 pixel gif with repetition or scale to fill the page.)
They have a different default gamma and they may show a different gray level.
(It bite me a long time ago. I made a gif that has the same RGB bacground than a webpage. In my pc it was fine, but in a mac they the border was very visible and the result horrible. My solution was to change the backgroung of the webpage from a RGB number to a 1 pixel gif with repetition or scale to fill the page.)