The difference
- CAN
- Electrical/data-link network carrying many ECU messages; raw identifiers are not self-describing.
- OBD-II
- Standard diagnostic request/response interface exposing supported PIDs through the diagnostic connector.
Why rates differ
Direct CAN can observe messages whenever the ECU broadcasts them. OBD-II commonly polls one parameter at a time, so adding channels can reduce per-channel update rate. Adapter, bus, ECU and app all affect results.
Do not assume channels
A CAN socket does not guarantee decoded throttle, brake or steering. AiM documents CAN/RS232/K-Line interfaces, but channel availability depends on the supported ECU definition. Generic OBD-II often provides RPM, speed and some temperatures; brake pressure and steering angle are rarely generic PIDs.
Safety
Passive logging still requires correct wiring and termination practice. Never transmit onto a vehicle network without understanding the consequences.