Seq WTF?

gistfile1.scala

scala> val things: Seq[String] = Seq("asdf")
things: Seq[String] = List(asdf)

scala> things match { case hd :: tl => "not empty" ; case Nil => "empty" }
res1: String = not empty

scala> val things: Seq[String] = scala.collection.mutable.ArrayBuffer("asdf")
things: Seq[String] = ArrayBuffer(asdf)

scala> things match { case hd :: tl => "not empty" ; case Nil => "empty" }
scala.MatchError: ArrayBuffer(asdf) (of class scala.collection.mutable.ArrayBuffer)
...

scala> Set(1,2,3)
res4: scala.collection.immutable.Set[Int] = Set(1, 2, 3)

scala> Set(1,2,3).toSeq
res5: Seq[Int] = ArrayBuffer(1, 2, 3)