Panelboard Busbars

NEC 408.3.E reads as follows

The phase arrangement on
3-phase buses shall be A, B, C from front to back, top to
bottom, or left to right, as viewed from the front of the
switchboard or panelboard. The B phase shall be that phase
having the higher voltage to ground on 3-phase, 4-wire,
delta-connected systems. Other busbar arrangements shall
be permitted for additions to existing installations and shall
be marked.

Have I missed the boat on this? A 3-phase system with a higer voltage on phase B than on phases A and C? I don't understand this. I thought a 3-phase system was, barring unbalanced impedances, balanced. What did I miss here?
 

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