Why doesn’t my iPad update to iPad iOS 16? It only updated to 15.7

Unlike iOS16 for iPhone, released 12th September (iOS 16.0.1 subsequently released 16th September), iPadOS16 is not due for release until sometime in October. 

However, iPadOS 15.7 was also released 12th September - incorporating essential security updates.

About the security content of iOS 15.7 and iPadOS 15.7 - Apple Support

While iOS16 is available for iPhone, iPhone owners have the option to just install iOS15.7 at this time.

More information about the forthcoming iPadOS16 can be found here:

iPadOS 16 Preview - Apple

iPad OS 16 has yet to be released.

Apple states that it will release in October.

You are quite welcome.