Hello,
Correct, we constantly improve our devices and use latest available factory firmware when manufacturing devices. Therefore devices, ordered few months later, might have different firmware version than previous ones.
Nonetheless, you can freely downgrade devices firmware to desired one, as long as new devices do not have any important hardware changes (for which old firmware versions was not compatible).
Take note that you can contact your Tetlonika sales manager and get your own custom device order code, which would allow you to order devices with specific firmware version and your configuration.