exercise 4.6 flatmap/orElse

[minnieshi]

the book says for flatmap (def flatMap[EE >: E, B](f: A => Either[EE, B]): Either[EE, B] = ???), it needs to promote left type to super type EE.
//when mapping over the right side, we must promote the left type parameter to some supertype, to satisfy the +E variance annotation. Similarly for 'orElse'

My question is : why do not we have to say B >: A, we do not need to satisfy +A ???... interestingly, the orElse signature has promoted the right type parameter to some supertype B (def orElse[EE >: E, B >: A](b: => Either[EE, B]): Either[EE, B] = ???).

I've tried to read covariance, subtyping, but still could not understand.