We always sized the circuit by the smallest conductor as you posted there so therefore you have 14-2 that is good for 15 amp circuits only but you also have 12-2 there as well that can be used either 15 or 20 amp circuit however as you have other conductor size you are restricted to the smaller conductors if there.
If you really want 20 amp circuit then you will have to run a new cable or conductors if in the conduit.
Merci,
Marc