Why does self.path = work instead of just path = ?

date_type.rb

class DateType
  attr_reader :entry
  attr_accessor :path, :error_state

  def initialize(entry)
    @entry = entry.to_s
    @path = ""
    @error_state = false
  end
  
  def prepare_path_that_does_not_update_the_instance_variables
    case
    when entry.downcase.include?("tra")
      path = "Transaction"
    when entry.downcase.include?("pro")
      path = "Process"
    when entry.downcase.include?("cre")
      path = "CreatedOn"
    else
      error_state = true
    end
  end

  def prepare_path_that_updates_the_instance_variables
    case
    when entry.downcase.include?("tra")
      self.path = "Transaction"
    when entry.downcase.include?("pro")
      self.path = "Process"
    when entry.downcase.include?("cre")
      self.path = "CreatedOn"
    else
      self.error_state = true
    end
  end

end

example_that_fails = DateType.new("tra")
example_that_fails.prepare_path_that_does_not_update_the_instance_variables
example_that_fails.path # => "", but I expected to get "Transaction"

example_that_works = DateType.new("tra")
example_that_works.prepare_path_that_updates_the_instance_variables
example_that_works.path # => "Transaction", why do I need to include self?