" The electron gain enthalpy of "O" is less negative than that of "S" ."

Assertion:For the elements O or F,the electron gain enthalpy is less negative than that of the succeeding element. Reason:Electron gain enthalpy becomes less negative as we go down a group.

Account fot the following: The electron gain enthalpy of Cl (Z = 17 )is more negative than that of Fluorine ( Z = 9 ).

Why is the electron gain enthalpy of O or F less than that of S or Cl?