I wonder if it's using a compass and odometry (distance) with dead reckoning. A strange choice when GPS is available, but it would account for the map moving in the car's direction and it not changing locations when it moved without rolling (ferry).

Almost certainly. GPS is not only easily jammed, but easily spoofed. If the car believed GPS instead of its own eyes, so to speak, then there’s significant potential that you’d see glitches more often. It could also be something of a safety risk when using its self-driving capabilities.

In addition to sibling's comments about jamming and self driving safety, there are many driving situations where there is no or poor GPS reception: tunnels, double deck bridges, double deck freeways, underpasses, urban canyons, actual canyons, etc. Also regional problems. The GPS constellation is in a 55° inclination, so if you are north of ~55N, or south of ~55S, you need a clear view of the southern/northern sky, respectfully, for reception, since there will be no overhead satellites.