Because as @tredre3 indicates, the GPU that is integrated onboard the CPU chip of the laptop is given 800MB to use as its frame-buffer space. Leaving 15.2GB for the OS to utilize.

All GPU's need memory to store the frame-buffer. For video cards, the memory is on the card, so no system RAM needs to be reserved. For integrated GPU's, part of system memory is reserved for the GPU (as it would otherwise have no frame-buffer).