Certain product (area or feature) developers may require or choose to implement new product features based on new Visual C/C++, or .NET language traits that will require newer MicroStation product dependencies to be installed on client machines and reflected in an updated product readme "Product Requirements" section.
The SDK will often internally "pre-wire" a newer toolset version and allow product developers to create and compile new code, perform testing, and eventually have it promoted into released products (with new client requirements shipped). Any upgrade issues are noted and shared with other development teams, where more areas of the product update code and build solutions accordingly. After the product has been updated to use the new toolset then the public SDK will state new requirements and set the default toolsets and 3rd party developers can to start to leverage the new features and traits of the updated toolsets in their development environments, code, and workflows.
So the product pre-requisite installs and the SDK requirements can differ for a period in time, though the SDK update requirements will specify a new toolset shortly after the whole product has been fully switched over to using the new toolset as a default.
SDK requirements should be updated more frequently than in the past in hopes developers can more quickly take advantage and realize what each significant toolset update has to offer.
HTH,
Bob