I think the dishing of the wheels is for strength.
When the cannon is moving it will sway to one side or the other, especially in turns.
The wheel on the inside of the turn is going to catch most of the side-ways forces.
If the wheel is dished, the hub tries to move out, pushing more on the spokes - tightening the whole wheel assembly.
With a straight (non-dished) wheel the sideways force will only cause the hub to pull away and loosen the assembly.
JMHBAO