How to get Android 7.1 on Moto G5?

As (partially) mentioned [here](https://android.stackexchange.com/q/195097/20449) , I've been using Moto G4 prior to purchasing Moto G5. On Moto G4 I went from Android 6.0 through 7.0 to 7.1. I assume that Moto G5 is a successor (next generation) of Moto G4. Correct me, if I'm wrong. If so, then why am I still on Android 7.0 or how to trigger immediate update to 7.1 or 8.0? I assumed that having newer phone = _newer or at least the same_ system version. I'd never expect newer phone having older system version than its one year older predecessor. What am I missing?