Water usage is an estimate. 3 factors are zone size, nozzle type, and watering duration.
Generation 2 can use a flow meter, when attached, we will use for exact usage.
The nozzle type and zone size (Advanced Zone Settings) can be both modified in the app to get a more accurate reading.
Here is a little more information on the topic:
If you search the forums for 'water usage' you should find some more similar postings, here is one for example.
Hope this helps.