Grade conversion for PhD application: Masters in France -> Phd in the US (Math)
I do not tick any of the boxes in your requirements, but as no one seems to, I will try and help. I am a graduate student in the US in a different field than yours, but have had a similar question regarding conversion. 

Admission committees acknowledge that different countries have different grading schemes, some schools are stringent and some have grade inflation. First of all, do not try to 'game' your GPA to reflect what you think would be the comparison. That would come back and bite you in the... back. Often the schools will ask for the original grades in your native scheme, and sometimes to translate it to a GPA using their own calculator - which you should use. I wouldn't write an explanation about it for two main reasons - (a) it doesn't look good, and mainly (b) you are not a source of authority and they have no reason to trust you on this. Even if they did it does not mean that you can give a proper conversion.

OK. What should you do? The best possible thing will be to get your class rank from your department and mention that in your CV/Resume. Unless your grades are abnormally low, it is my understanding that the CV will be looked at - especially if you have a decent GRE, recommenders that are semi-known, or decent experience. Rank beats GPA. 
If that is not available, just assume that foreign students are given somewhat more leeway in the GPA department and your profile will gain a fleeting look if it is not all borderline.

Good luck!