ΔfHo for white phosphorus has been taken to be zero, whereas black phosphorus is the most stable allotrope of phosphorus and this is the only exception.
The reference standard state of white phosphorus is taken despite the fact that this form is not the most stable allotrope but simply the most reproducible form of phosphorus.
Standard enthalpy of formation is the change of enthalpy during the formation of 1 mole of the substance from its constituent elements in their most stable forms (Standard states 1 bar and usually 298 K).
The standard enthalpy of formation of elements in their pure state at 1 bar and usually temperature of 298 K is assumed to be zero.
Out of known allotropes of phosphorus black phosphorus is the most stable one but the standard enthalpy of formation of white phosphorus is zero
The exception comes into picture due to the difficulty in formation of black phosphorus.
White phosphorus is relatively easier to form so the standard enthalpy of formation for white phosphorus is zero instead of most stable black phosphorus.