[jhowell]

Yes, but using the gray card you are compensating for your light sources. In this case the light source for the white paper is different from the (internal) light source of the kindle.

ETA: For example, if I take two photos one with very warm (red) room illumination and one with very cold (blue) and then I compensate so that the paper is pure white in both cases then the kindle will appear very differently in each picture.