Enthalpy change question

Why is it that in when calculating the enthalpy of combustion you use 'bond enthalpies of REACTANTS - bond enthalpies of PRODUCTS'

but in calculating the enthalpy change of formation you use 'bond enthalpies of PRODUCTS - bond enthalpies of REACTANTS'?

(the other way round)

thanks sm

Why is it that in when calculating the enthalpy of combustion you use 'bond enthalpies of REACTANTS - bond enthalpies of PRODUCTS'

but in calculating the enthalpy change of formation you use 'bond enthalpies of PRODUCTS - bond enthalpies of REACTANTS'?

(the other way round)

thanks sm