Tuple2 and maps

I've been reading the scala eBook and found an example of adding a key value pair to a map:

map += (1 -> "Some String")

it seems that the (1 -> "Some String") results in a Tuple2, however the (1, "Some String") also results in a Tuple2, as follows:

scala> 1 -> "Some String"
res8: (Int, java.lang.String) = (1,Some String)

scala> res8 getClass
res10: java.lang.Class[_] = class scala.Tuple2

scala> (1, "Some String")
res11: (Int, java.lang.String) = (1,Some String)

scala> res11 getClass
res12: java.lang.Class[_] = class scala.Tuple2

The problem I have is understanding why map += (1 -> "Some String") works fine, but:

scala> map += (1, "Some String")
<console>:6: error: type mismatch;
 found   : Int(1)
 required: (Int, String)
       map += (1, "Some String")
               ^

gives me an error.

Thanks

Lorenz