A front projection system is very different, fundamentally, than a flat panel TV. A front projector is sending light to a passive screen from the front. This has two very important implications: 1) the only way you can get "black" is by the absence of light, and 2) any light hitting the screen also reflects back into the room and causes the potential for true 'black' to decrease. On top of that, a third factor would be that you are also measuring light output from a reflected surface, rather than from the emitted source itself.
Discuss "Why are TVs Typically Brighter than Front Projectors?" here. Read the article.