What is the difference?

That is objectionable in the free offer of the gospel, or well-meant gospel offer, is not the teaching that the church must preach the gospel to everyone and must call all hearers to faith in Jesus Christ. But the error of the doctrine of the offer, and the reason a Reformed man must repudiate it, is its teaching that the grace of God in Jesus Christ, grace that is saving in character, is directed to all men in the preaching of the gospel. Inherent in the offer of the gospel is the notion that God loves and desires to save all men; the notion that the preaching of the gospel is God's grace to all men, an expression of God's love to all men, and an attempt by God to save all men; and the notion that salvation is dependent upon man's acceptance of the offered salvation, that is, that salvation depends upon the free will of the sinner. - David J. Engelsma, Hyper-Calvinism and the call of the Gospel