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The site only tracks Ubisoft Challenges, which are separate from the achievements. There's a tab specific to the challenges inside the Ubi Connect app as well - it should be matching up with the list displayed there.
A lot of older games no longer have challenges - they used to but Ubisoft removed them all when they relaunched the system. AC Origins is one of those - the full list of games with challenges can be found here: https://www.exophase.com/platform/uplay/