LG VX-8100

This may show my lack of knowledge of cameras, but I've noticed (with my phone on the exact same settings) that one picture I took in one place came out clear as anything and another one I took later (in another place) came out a little grainy looking. The White Balance is set on AUTO, resolution 640x480, night mode off, and brightness is set in the middle). Could it simply be the lighting that would cause one to come out really clear and the other to come out grainy? Thanks!