"45. God's will [by way of decree--CD] determines all things without exception: the greatest, the least, the contingent, the necessary, the free. The Scripture shows this with respect to all kinds of things, e.g., Jesus Christ is to be glorified and the church saved through him, Pss. 2, 110:4, 40:7, 8, 9; Heb. 7:16, 21; Eph. 5:25; and 2 Tim. 1:9. Concerning Pharaoh, Exod. 13, where God disposed all things that he might move Pharaoh to follow and overthrow the people of Israel, nay, he hardened him that he might follow them, still Pharaoh and Israel worked freely. Likewise in the selling of Joseph, wherein all things happened freely and contingently, God determined it according to his own will. It is the same with the very heart of man, Ps. 33:14, 15; 1 Sam. 10:9, 26; Prov. 21:1. With a man killing another man by chance, Exod. 21:13. With the lot cast into the urn, Prov. 16:33. With sparrows falling to the ground, with all the hairs of a man's head, Matt. 10:29, 30. With the lilies, the flowers, and the grass of the earth, Matt. 6:28, 30. And finally with all created things, Job 38; Ps. 104; Isa. 45:7; Jer. 14:22. (William Ames, The Marrow of Theology, pp. 98-99).
"49. The will [decree--CD] of God does not imply a necessity in all future things, but only a certainty in regard to the event. Thus the event was certain that Christ's bones should not be broken, because God willed that they should not be. But there was no necessity imposed upon the soldiers, their spears, and other secondary causes then present" (William Ames, The Marrow of Theology, p. 99).
Additional Reformed writers (e.g., W.G.T. Shedd; William Perkins; Loraine Boettner) have likewise made this peculiar distinction between "necessity" and "certainty" in the decree of God. Others (e.g., Johannes Wollebius) who have affirmed a "'necessity' in all future things" make distinctions between necessities that are "coercive" with those necessities that are said to be "merely immutable." So despite using different terms they are basically saying the same thing -- which, in effect, is this:
If God brings these immutably certain future things about in this way, then He cannot find fault with them. To focus upon the Ames' quote above. If Divine "necessity [is] imposed upon the soldiers, their spears, and other secondary causes then present," then God cannot find fault with any of them. For (after all) who has resisted His will (Romans 9:18-19; cf. Isaiah 10:5-15)?