Why? It's not been rewritten from scratch. Something that affects previous versions could reflect in the latest version. (Note, I don't know what the vulnerability is).
The bug in question is in Microsoft XML Core Services - code that was written and deployed in Windows XP SP3.
A remote code execution vulnerability exists in the way that Microsoft XML Core Services handles objects in memory. The vulnerability could allow remote code execution if a user views a website that contains specially crafted content. An attacker who successfully exploited this vulnerability could take complete control of an affected system. An attacker could then install programs; view, change, or delete data; or create new accounts with full user rights. Users whose accounts are configured to have fewer user rights on the system could be less impacted than users who operate with administrative user rights.