The analysis of earthquakes is in constant flux.
Preliminary data is just that, preliminary data.
While automated analysis systems are very quick and make excellent approximations of events, they are not sufficiently accurate enough just yet to make final seismic results. The benefit of these systems, however, is extraordinary because the will alert people that an earthquake has occurred event, though the data is preliminary. Better approximations of the earthquake perimeters comes after review by a trained person in seismic data analysis, usually a geophysicist or seismologist. As new data is analyzed, either within minutes, hours, or days, of the event, the results may change and if posted, another e-mail alert bulletin is issued. As the seismic data is processed at these networks, it is retrieved and sent out as an e-mail Earthquake Alert Bulletin.