On plant labels, the height stated is often the height after five or ten years, not the ultimate height. Unfortunately, a tree doesn't stop growing at ten years old.
How much growth does it make in one season? Even if it grows just 6 inches per annum, that's 5ft in ten years. If you want to keep it in check, you'll need to trim it annually, not cutting into the old wood. You can't leave it alone for years and then lop off all the excess growth in one go, it doesn't work like that.