It's an average of city (55%) and highway (45%) and a fudge factor. If you wish to read more, go here Detailed Test Information and here for EV/PHEV specific info https://www.fueleconomy.gov/feg/pdfs/EPA%20test%20procedure%20for%20EVs-PHEVs-11-14-2017.pdf
One can find the actual detailed results for any specific car here, under the annual 'Datafile' listing: Download Fuel Economy Data Before averaging and fudge factors, your 2014 Prius tested and measured out as: City Unadjusted FE: 71.6510 MPG Hiway Unadjusetd FE: 69.4488 MPG I don't see entries for the other three tests (maybe they are already rolled into the above two?). Do note that the 'fudge factors' Tideland mentions bring these Unadjusted numbers down sharply to produce the final adjusted and rounded EPA rating number.
On actual flat roads in actual conditions driving a steady speed the EPA crossover fuel economy is the most important. If it’s low AKA 55mph or slower where instantaneous economy matches EPA you will have trouble matching EPA If it’s high 65mph or faster you will easily beat EPA, sadly car makers don’t have to publish this or economy at low temperatures
