Hi Reda,
Not exactly. Google doesn't determine rankings just by looking at how well optimized a site is for individual words. They look at many other factors both on and off the site, so it's really hard to say why you're ranking slightly higher for one.
To be honest, I wouldn't stress over individual keyword rankings too much. You'll end up driving yourself insane. With so much personalization in search results today, what you see in search results isn't necessarily what someone else will see. I think its better to look at overall trends and just make sure you're moving in the right direction.
Tim