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).

siri is honestly only good for basic question/answers. like "how is the weather going to be later" or "play X song", or "how old is X". i wish they integrated LLMs into personal assistants to truly have a conversational type experience