For some words, it may simply be a matter of a different spelling: acteur, automatique, descendre, photographie.
For other words, there may be a connection with a related but little used term in American English: arbre (arbor) for tree, cuisine for kitchen.
A few words in French are a straight borrow from English: le weekend.
But, not infrequently, even my trés knowledgeable professeur has no explanation for why some words or phrases are used. Her explanation - "It's French."
I recently came across a similar dilemma in British English. While proofreading a client document, I noticed they seemed inconsistent in the spelling of the word licensing (American) / licencing (UK).
The explanation they provided is that, "In British English, when used as a noun, the correct spelling is 'licence'... when you add the 'ing' to 'licence', it becomes 'licensing'."
That's a new one to me. Certainly looks awkward to have licence and licensing mixed on the same page. I guess "It's British."
Same goes for commas and periods outside the quotation marks.
Please don't get me started on Canadian English.