Limited / Particular Atonement

We have a high view of Christ's victorious work on the cross. His atonement was sufficient, efficient and accomplished all that the Father appointed Him to do. There is no failure, no weakness and no possibility of defeat in His work as the eternal God-Man suffering on the cross for sinners. We refuse to believe that anyone is in hell for whom Christ died. The question then is this: Why is it referred to an limited atonement in the Tulip scheme?