Why does irb echo the right hand side of an assignment instead of the return value in the case of a setter method?

It seems surprising that in all other instances irb will echo the return value of a method. Why does assignment via a setter behave differently?

I'm using Ruby 2.2.2.

irb(main):001:0> def x=(value); puts "puts_from_x"; "returned_string_from_x"; end
=> nil

irb(main):002:0> self.x = 3
puts_from_x
=> 3

update

It has dawned on me that it echoes the rhs because that's the actual return value. Why is this?