Why do some Audi A3 models have a start-stop system while others don't?

I've noticed that not all Audi A3 models have a start-stop system, but I can't figure out why. Can someone explain the reason for this difference?

Some Audi A3 models come equipped with a start-stop system while others do not due to variations in the vehicle's engine size and fuel efficiency. Smaller engine sizes often have better fuel efficiency and therefore do not require a start-stop system.

The inclusion of a start-stop system in Audi A3 models is also dependent on the specific trim level and features chosen by the buyer. Higher trim levels typically come with more advanced features such as a start-stop system.