That's a great question. I have no idea why you would pay for more because there isn't an overage. You can pay $30 per month and if you use 4 GB, you pay $40 instead.. So I'm not sure why they would even differentiate that.
When I think about it some more, maybe it means you don't actually pick a data tier and instead just pay as you go? Although I believe they specifically stated that you can choose differing levels of data to initially pay for, so I guess that's not it.
For me, sometimes I end up using a lot of data in a month, but most months I do my heavy lifting on Wifi. This plan really would be perfect for me. T-Mobile throttles you when you pass your point (no good), Verizon and AT&T charge you big bucks for going over (no good), and Sprint has affordable unlimited data but most of the time it sucks (no good). Google Fi gives me the best combination, no throttling, no overages, two networks combined for coverage, and a refund on the data I don't use.
No one else that I'm aware of offers this combination of features (no overages, no throttling, only pay what you use). It's a killer combo to me.
Can't wait to try it out! Now I'm thinking of swapping my Note 4 for a Nexus 6...