1.day Ruby's case operator === behaved badly

This is a follow-up to a previous post about the Ruby case operator, ===.

I previously understood that given

    a === b

when a is a class, the result would be b.class.ancestors.include?(a)

when a is an instance, the result would be a == b

This proves true for many examples.

>> 3.class.ancestors.all? {|a| a === 3}

=> true

So I was very suprised to find this:

>> 3.days.class

=> Fixnum

>> Fixnum === 3.days

=> False

>> 3.days.class.ancestors.include?(Fixnum)

=> true

here's the method days:

def days   ActiveSupport::Duration.new(self * 24.hours, [[:days, self]]) end

And, in fact:

>> ActiveSupport::Duration === 3.days
=> true

I'm not clear on why this happens. I'll update this post as it becomes clear.