I believe it's because it is sensing movement but may not be 100% accurate. I have 2 vehicles that I drive on a regular basis and the activity that is registered varies quite a bit between the 2 on the same roads and same trip.
1 is a 98 Mustang GT Convertible which has been lowered and has low profile tires. It's stiff and you feel every bump in the road. I'm also shifting gears and cornering hard in it too. The activity registered is quite a bit more than my truck.
The other one is a 11 Silverado Crew Cab which has a much smoother ride and I don't drive it like I stole it.