@Raining2017 - welcome to the Rachio community. There are numerous threads and discussions on the board highlighting issues with using number of heads and emitters for calculating water use.
Rachio uses the precipitation rate (not GPM flow rate) from the nozzle type and the area of the zone along with the zone run time to estimate water used. If the area size isn't changed from the default 1,000 square feet under the Advanced setting, and there really aren't 1,000 square feet in the zone, this leads a majority of the discrepancy.
Accurate usage can be estimated by performing a catch cup test, creating the appropriate custom nozzle (https://support.rachio.com/hc/en-us/articles/115010542168-How-do-I-create-a-custom-nozzle-#6) and setting the area for the zone. Others have used this process to come very close to their observed water meter readings. Another option is to install a flow meter.