Any terrorist attack in France will almost certainly happen in the center of one of the major cities, most likely Paris. So if you live in the center of Paris your micromort should be much higher, and if you live in a small village it will be much smaller. 
Also the micromort calculation quoted in the article is for being killed in a single terrorist attack in which 153 people die. If you think there are likely to be several such attacks over the next few years then that increases the micromort value higher.