A 710 is indeed a little low compared to your cohort. It qualifies you but it's not as high as we like to see for a 2+2 candidate; the averages are incredibly high for those accepted students.
On the French grades, we have understood from our other clients that a 14 to 15.9 is a "bien" or B grade, and a 16 to 20 is a "tres bien" or TB grade. It sounds like you're hitting lower than you estimated - a 14.5 is unlikely to translate to a 3.7 on the U.S. system. You should always report your marks in the system that your institution uses.
You are getting confused
On the baccalauréat 16 does correspond to TB and 14 to Bien. I scored a 16.57 = TB on mine
In upper studies though, the system gets really confused, mixed between university (faculté) and Grandes Ecoles... with different grade systems
As a matter of fact, London School of Economics for example might ask for a 13/20 for someone at "faculté" to get into master's, while it only asks for 12/20 from someone in "Grande Ecole"..
Weird, confusing French system :D
I was thinking that my grades while at Hopkins might help HBS assessing my grades.. I took 300/400 level classes (except for my Russian classes, which started 200) and got a 3.76 GPA