How much difference are we talking about? AFAIK, the standard spacing is 44mm, and usually you'd just be dealing with "manufacturing tolerances". Back when, there was a special "narrow rail" B17 with a matching Campy post, but that was decades ago. And now that you mention it, one of my B17s seems to be a few (3-4) mm wider than the cradle on my FSA post. But I just squeeze the rails together til they fit. So, no, it's probably not another case of "French standards".

SP
Bend, OR