Unexpected Ruby behavior when instantiating two instances on one line

I created a class that generates a different name for each instance, but a test fails unexpectedly when instantiating two instances in one statement.

Here is the class.

class Robot
  attr_accessor :name
  @@current_name = 'AA000'
  def initialize
    @name = @@current_name
    @@current_name.next!
  end
end

Here the class behaves as expected

irb(main):009:0> Robot.new.name
=> "AA001"
irb(main):010:0> Robot.new.name
=> "AA002"

Here is the unexpected behavior, I was expecting false. This code is in a test on an exercise I am trying to get passing, so I can't change the test.

irb(main):011:0> Robot.new.name == Robot.new.name
=> true

Checking the object_id reveals that two different instances are being created.

irb(main):012:0> Robot.new.object_id == Robot.new.object_id
=> false

Why is Ruby doing this, what should I do to fix it & assuming there is a term for this, what could I have typed into search to find answered questions about this.