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.