Why every second word in this article is "AirGradient"? Most important/expensive sensor is from Senseair, controller from Wemos, etc. I've made same monitoring

Why every second word in this article is "AirGradient"? Most important/expensive sensor is from Senseair, controller from Wemos, etc. I've made same monitoring solution using same components following some years old tutorial and have never heard about "AirGradient".