Why/How does this scala contravariant example work?

This code seems to set up a contravariant example: FruitBox can take in Apples or Oranges.

class Fruit(name: String) { }
case class Apple(name: String) extends Fruit(name) {}
case class Orange(name: String) extends Fruit(name) {}

class Box[-T] {
  type U >: T
  def put(t: U): Unit= {box += t}
  val box = scala.collection.mutable.ListBuffer[U]()
}

  object test {
   val FruitBox = new Box[Fruit]
   // Fruit Box takes in everything
   FruitBox.put(new Fruit("f1"))
   FruitBox.put(new Orange("o1"))
   FruitBox.put(new Apple("a1")) 

   // and Orange Box takes in only oranges
    val OrangeBox = new Box[Orange]
     //OrangeBox.put(new Apple("o2")  ==> Compile Error that makes sense
    OrangeBox.put(new Orange("o2"))

   // Contra-variant nature is also demonstrated by
     val o: Box[Orange] = FruitBox 
  }

That is all fine...but why does it work ? specifically: 1. When the FruitBox is initialized, why does the "type U >: T" not constrain it to supertypes of Fruit? In spite of that constraint, the FruitBox is able to put the subtypes if Fruit ( oranges and apples )...how?