why two numbers in android

I'm trying to understand why android uses two sets on numbers , platform version and API level. API Level, documentation states 开发者_如何学Cthat "Each Android platform version supports exactly one API Level", does that mean, Google can provide a new(improved) platform, but still decides to go with earlier API level thus not making any change in Java API's for the developers in the new platform. Is my understanding correct? Also, what changes happen when platform version changes?

You are correct to think API and platform are not interrelated.

Platform changes would most likely be performance improvements, improved security, or better framework.

Whereas API versions can be introductions of new or improved interfacing methods.