Some 9920's came with 2 wires per phase. There is no particular reason for it. if you look at the assembly of the controller, you could see that it would be easier to use two wires, instead of one. (if you have a controller with 1 wire per phase, the wire splits up at the two boards that are in the controller)
I personally like the 2 wires per phase because of this.
Look, i have one like that too. I have got another one, that has 1 wire per phase,