Common mode chokes increase the Common Mode Rejection Ratio (CMRR) by providing high impedance for common mode signal (i.e., when the bus is in a recessive state with no differential voltage between CANH and CANL) and providing low impedance for differential signals (i.e., when the bus is dominant). Chokes are common on CAN busses and can implement filtering on the common mode signal without adding a lot of distortion. The coils are usually matched fairly well. Like RC and LC filters, any mismatch in the inductance of the coils can cause distortion on the differential signals. Another thing to be aware of is that the inductance of the coils and the capacitance of the PCB could create a tank circuit and cause oscillations.