2.5 is not the version number, it's the generation of the underlying model architecture. Think of it like the trim level on a Mazda 3 hatchback. Mazda already has the Mazda 3 Sport in their lineup, then later they release the Mazda 3 Turbo which is much faster. When they release this new version of the vehicle its not called the Mazda 4... that would be an entirely different vehicle based on a new platform and powertrain etc (if it existed). The new vehicle is just a new trim level / visual refresh of the existing Mazda 3.

That's why Google names it like this, but I agree its dumb. Semver would be easier.