It may just take too long and waste too many resources in the interim to rely on the self-equilibrating forces of the economy. Policymakers will therefore have to take into account, in practice, both objectives in their policy actions. Still, a meaningful difference in the interpretation of the two objectives remains.
A central bank can achieve the inflation target, with considerable precision, in the long run, meaning on average over a period of years. But it cannot be expected to maintain the economy continuously at full employment. The full employment objective might therefore be better interpreted as an output stabilization objective.