Many gym leaders as well as Elite 4 members as well as Champions do tend to always carry a Pokemon that is not intitled to their signature type.
It is very well possible that Volkner is using because it assists him with his job as the main electrician in Sunyshore City. However take note this has never been confirmed. However it should be noted that the Ambipom he used in his gym team did carry Shock Wave which is an electric type move as well as his Octillery having Charge Beam, making them semi-viable as a gym Pokemon for an electric type gym leader.
Also, technically no one has ever said that Gym leaders are prohibited to use any type other than the type they specialize in.
Champion Steven also uses many fossils in his team as they are strongly related to his passion of collecting stones.
Ambipom is a very handy Pokemon capable of holding multiple things at once, so I wouldn't be surprised if he would ever use it as an assistance and later use it in his gym match.
Shortly, there isn't really a reason to him using an Octillery and an Ambipom other than the fact that they could simply be Pokemon he likes, or Pokemon he often uses outside of his gym.