Just to echo what RooJ said; the images we see when we are awake are processed through the brain before we see them so when we dream our brain brings all the things we see back up so it is possible to see something 100% the same as RL. Also many people seem to exp. increased clarity in some dreams which i guess is the brain sharpening the image for you due to some reason, while others have hazzy dreams which could mean the memory retreval isnt perfect, the other explanation could be how good your recall is of the particular dream.

Just another point on realism in dreams from personal exp. i find that you can exp. everything you can in RL. Now this sounds freaky but you get used to it, it does however in my case fluctuate in how realistic the dream is. Something that springs to mind after last nights strange LD is exp. pain in my dreams. I have felt pain a fair bit in dreams, this was really bad at first as i would wake up and have to check my self over for injuries and that sort of thing as i would stress out about it. These days i just take it for granted, pain is something that comes from the brain interacting with its enviroment so if the environment is a LD then i guess it makes sense that i can hurt in it (you havent been pissed off till you feel like you have paper cut yourself in a dream and wake up due to it).