Storj calculates this based how how many hours you store data. 1 GBh is 1 GB stored for 1 hour. Or 2 GB stored for half an hour. The satellite keeps accurate track of how much data is stored for how long during the month. If you divide that total GBh number for the entire month by the number of hours in that month, you get GBm, which comes down to how much you node has stored on averag over the entire month.
Now the estimator throws all of that out of the window and just looks at how much you had at the start of the month and how much at the end and just assumes the average across the month is halfway between those 2. It’s only an estimation after all and growth is usually fairly linear, in which case this would be very close to reality.
This is because the satellite aggregates the GBh values roughly twice a day. But the periods can fluctuate. So on some days the node receives the data for 20 hours and then the next day for 28 hours for example. The total will always add up correctly though.
I actually already mentioned this in my previous post in passing. But I can add that my earnings calculator as of now is the only way to get this data as it isn’t displayed on the dashboard and was removed from the storagenode API. You can try it here: Earnings calculator (Update 2023-12-05: v13.1.0 - Now with support for different payouts per satellite - Detailed earnings info and health status of your node, including vetting progress)