Remember, a critical part of Google Fi is that it's supposed to automatically switch your voice and data to WiFi, and that Google has rigged partnerships such that they have lots of WiFi available for you to use.
So, if you currently use 5-10 GB/mo on one carrier, the theory is that you'll use less - possibly significantly less - on Google Fi.
That's SIGNIFICANTLY different from what you can see reported about how you manually configure the WiFi you have used today.