Perhaps the reason for that is we do not understand it either, at least not completely. It is a paradox. But we believe it is biblical, so we believe it is true, and we do not think it is a contradiction.
To refresh you a little on what you may already know, a contradiction is if a statement is both true and false at the same time. Some examples of contradictions are, "God planned our sin, and God did not plan our sin," or, "We are responsible for our sin, and we are not responsible for our sin." Both of those statements are contradictions, but neither is what Calvinism teaches. Calvinism teaches, "God planned our sin, and we are responsible for our sin." It's paradoxical because in our normal way of thinking, if we plan something, we're the only ones responsible for it. But the only way it's a contradiction is if you make the assumption that, "The planner is the only responsible party." The Bible indicates this is a faulty assumption, therefore, "God planned our sin, and we are responsible for our sin," is not a contradiction.
Wayne Grudem compared it somewhat to Shakespeare. We would not hold Shakespeare accountable for King Duncan's death. Macbeth murdered King Duncan, but Shakespeare wrote the story. It's not a perfect comparison, of course, but it is helpful as an insight into how the responsible party can be someone other than the planner.
