It might be a European vs. American cultural thing because week number is much less used in America, and Siri was developed by Americans. Maybe this is more of

It might be a European vs. American cultural thing because week number is much less used in America, and Siri was developed by Americans. Maybe this is more of a localization failure than a generic functionality failure. If it gave you an answer that seemed wrong, well, there are also several slightly different definitions of week number, so it could be that it chose one that wasn't the one you expected (maybe also a localization failure).