They are 2 analog dials and emulated as such, though MAME provides digital inc/dec keys that rotate the dials by a fixed amount per frame (the cursor keys in this case).

On the real Blitz, rotate the dial faster to scroll through the settings faster. And if you rotate the dial a tiny bit (like tapping the key in MAME for a brief moment), it hasn't moved enough yet to go to the next setting.