Reloadron, I think you might be thinking at the boundary between split phase and poly phase. Poly phase requires the voltage lead or lag each point by some non zero non opposing degree, in a split phase system they're 180 degrees out of phase which mean's even though the phase between two points is different it's not truly poly phase. It is often used when simple equal valued + and - outputs are desired from a power supply. MANY transformers have more than two taps, they don't always need to be centered. Sometimes they're sense coils or secondary outputs.