Every vessel crew has to enter the AIS message manually, which often leads to unintentional or accidental errors. Often incorrect MMSI numbers like 123456789 are entered. In China, one MMSI number is often used for many different vessels. This means that several ships all over the world's oceans send the same MMSI number at the same time, so they cannot be distinguished from each other without closer examination. It is difficult to identify their tracks, because instead of appearing as separate ships, they appear as one ship with one track, jumping back and forth across the ocean at impossible speed.
Instances of spoofing represent less than a quarter of one percent of the AIS signals. Certainly not all spoofing represents illegal or undesirable activity, but it is unquestionably one method illegal fishers use to conceal their identity and their behaviour.
FleetMon tries to filter out jumps from ships and clean up the system.