Accessor confusion with Array Union

accessor_union.rb

class MyClass
  attr_accessor :values, :uniq_values

  def initialize(value)
    self.uniq_values = ['default_value']
    self.values = ['default_value']
    copy_value(value)
    add_value(value)
  end

  def copy_value(value)
    uniq_values |= value
  end

  def add_value(value)
    values << value
  end

  def run
    puts "unique_values: #{uniq_values}"
    puts "values: #{values}"
  end
end

obj = MyClass.new('poop')
obj.run

# Expect 'unique_values' and 'values' to be the same
# OUTPUT:
#  unique values: ["default_value"]
#  values: ["default_value", "poop"]