They probably put the +/- 5% there because of the engine they are using. This particular engine from Toyota is rated at 170hp (Corolla XRS), 173hp (Pontiac Vibe), 180hp (Celica, Matrix), and 190hp (Euro Celica). I guess they all have varying setups to make the horsepower ratings differ and Lotus isn't sure how well the engine will perform in the Elise.