The best bet with projections is when a site will give you an average of projections from at least 3 sources.
But, the reality is what murphysxm said, and take the projections with a grain of salt. No one saw Arian Foster, Terrrell Davis, or Marquez Colston doing what they did their rookie seasons. But they did it.
And of course, injuries come into play when the real season hits.
"We couldn’t do diddley poo offensively, we couldn’t make a first down, we couldn’t run the ball, we didn’t try to run the ball, we couldn’t complete a pass - we sucked." ~ James Ernest Mora