God can change His mind, God chooses not to see everything, I don't know how it works exactly but God knows everything but doesn't know everything, here's a verse to ponder:
Psalms 11:4 *¶The LORD is in his holy temple, the LORD'S throne is in heaven: his eyes behold, his eyelids try, the children of men.
Also the two agreements (covenants) were different, one was if you sin you die, the other one was if you believe you live. one was based on man getting to God the other God getting to man. There is so many differences between the two, the law was only a shadow of what is to come.
I'm feeling more lazy than usual, here's something more to ponder that I ccp:
Law always has a place and work distinct and wholly diverse from that of grace. Law is God prohibiting and requiring; grace is God beseeching and bestowing. Law is a ministry of condemnation; grace, of forgiveness. Law curses; grace redeems from that curse. Law kills; grace makes alive. Law shuts every mouth before God; grace opens every mouth to praise Him. Law puts a great and guilty distance between man and God; grace makes guilty man nigh to God. Law says, "An eye for an eye, and a tooth for a tooth"; grace says, "Resist not evil: but whosoever shall smite thee on thy right cheek, turn to him the other also." Law says, "Hate thine enemy"; grace says, "Love your enemies, bless them that despitefully use you." Law says, do and live; grace says, believe and live. Law never had a missionary; grace is to be preached to every creature. Law utterly condemns the best man; grace freely justifies the worst (Luke 23:43; Rom. 5:8; ! Tim 1:15; 1 Cor. 6:9-11). Law is a system of probation; grace, of favor. Law stones an adulteress; grace says, "Neither do I condemn thee: go, and sin no more." Under law the sheep dies for the shepherd: under grace the Shepherd dies for the sheep.