Its also depends on the configuration of the video card, if we are talking about Nvidia, maybe you have your SLI configuration disable, so this make the GPU in position 0, your main working GPU, that might explain the difference of temperatures.
if you also has an auto detect SLI it will only work when the strees is out of some boundaries of the single card, for example, if you play a videogame like crisys 3 and you have a GTX 580M in SLI configurated to enable SLI when required, then it will enable as soon as you run the game.
That could explain the differences of 10 degres between one card vs another card.
and more simple explanation could be laptop design, if thats the case i recommend tou but a cooler for your laptop so that way you can regulate better the processors temps and GPUs also.