Scala对象在特征之内

trait SimpleTrait {

  def setString(s: String): Unit = {
    InnerTraitObject setString s
  }

  def getString(): String = {
    return InnerTraitObject getString
  }

  object InnerTraitObject {
    var str: String = ""

    def setString(s: String): Unit = {
      str = s
    }

    def getString(): String = {
      return str
    }
  }
}

然后

class SimpleClass extends SimpleTrait{
 /// empty impl
}

和：

object App {

  def main(args: Array[String]): Unit = {

    val a = new SimpleClass();
    val b = new SimpleClass();

    a.setString("a");

    println("A value is " + a.getString());
    println("B value is " + b.getString());
  }
}

我想看看

A value is a
B value is a

但我得到了

A value is a
B value is

我的问题是：
如果对象是单身,那么为什么如果我把它变成特征,那么它的行为就像普通对象？