Microsoft has detailed its policy of patching in-house discovered bugs silently and tried to answer the most frequently asked questions regarding this procedure.
The fact that Microsoft doesn't disclose all patched vulnerabilities in Security Bulletins is not a secret. This was admitted by the company in 2006.
This somewhat controversial policy applies to bugs discovered during the "Hacking for Variations" (HfV) process, which aims to limit the number of similar flaws in a product.
When the company receives reports of a vulnerability, it also inspects the source code for similar bugs and runs a plethora of tools, including fuzzers, against the vulnerable component.
Any flaw discovered in this way is considered a variant of the originally reported vulnerability and it doesn't get publicly disclosed, nor does it receive a CVE identifier.