As mentioned, it is one leg of the 3 phase. It is not used to neutral. The fact that it has a reference to neutral is besides the point.

For example, say you hooked up three single phase pots in a 4 wire delta configuration. Each of the pots forming the "high" leg are conventional single phase pots, so they each have an X2 bushing that is simply not used. But if you took a voltmeter and read from them to neutral (or any other phase) you would still get a reading. It's just not a usable voltage.

Off topic a little. Another reason the delta's are being phased (no pun intended [Linked Image] ) out is that they are more susceptible to ferro-resonance problems. As distribution voltages go higher and higher, this has become more of a problem.