They may be measuring engine braking. Many trucks use Jake brakes to slow down and save their regular brakes, especially on downhills. A regular engine brake on

They may be measuring engine braking. Many trucks use Jake brakes to slow down and save their regular brakes, especially on downhills. A regular engine brake only slows you down a little, and isn't nearly traction-limited. I imagine that an electric regenerative braking system could be friction-limited.