Jackie Caplan-Auerbach, a seismologist at Western Washington University, explained it this way in a series of Twitter posts:
“Location algorithms can sometimes yield an incorrect location, though. This is because they are somewhat tuned to what they expect to record. For example, a seismic network in California is focused on recording CA quakes, so the algorithm starts with that assumption.
“...Luckily, humans are smarter than computers, and we always have humans review the algorithm’s output. Sometimes...and this is actually pretty rare...we find a phantom quake and delete it from the record.”