AIS data is notoriously unreliable. The AIS system is meant to prevent collisions between ships, and for that purpose it works well. There are a number of large multinational firms as well as government that subscribe to the AIS satellite and terrestrial coverage. There are two constellations of civilian satellites I am aware of that can pick up the AIS transmissions; there are probably more that are military/intelligence.
I worked extensively with a large AIS dataset for several years. We often times would see vessels with incorrect IMO numbers, duplicate vessels (in different positions) or even vessels located in the middle of Africa or Siberia. Part of this is due to how the transmissions arrive at the satellite; the transmissions can 'collide' and the satellite gets confused as to the true transmissions that are arriving simultaneously. This means that using the AIS data requires several layers of data cleansing.
Speed was notoriously unreliable on AIS and does not correspond with distance traveled for a vessel track. We would sometimes see vessels traveling at an indicated 100kt; obviously impossible. We would ignore speed and compute a vector based on the last two positions and elapsed time. Destination is also unreliable on AIS as it is entered by the crew and often times they will forget to change it after leaving a port.
Finally, AIS transmitters are deliberately turned off around the horn of Africa, mainly to avoid detection by pirates.
Spoofing an AIS signal would be trivial for anyone as well who wanted some type of 'civilian' cover.