Well it kinda goes like this, people become familiar with 3 phase and associate it with 3 wires, then they later see 2 wires and say hey it's 2 phase... but that is not how it works, nor what it means... with 3 phase their are 3 sine waves 120 degrees apart, if one has a 3 channel oscilloscope, this can be see as the some wave between legs a to b, leg b to c and legs c to a... with only 2 wires, we have only one sine wave or one phase, the center tap only provides a voltage divide by 2 function, stiil just one sine wave, because if we have an equal numbers of devices on the leg a to center tap and leg b to center tap, we have no current flow through the center tap... It's just one phase...

Ron