Ruby - Lexical scope vs Inheritance

This is a continuation this original SO question: Using "::" instead of "module ..." for Ruby namespacing

In the original SO question, here is the scenario presented which I'm still having trouble understanding:

FOO = 123

module Foo
  FOO = 555
end

module Foo
  class Bar
    def baz
      puts FOO
    end
  end
end

class Foo::Bar
  def glorf
    puts FOO
  end
end

puts Foo::Bar.new.baz    # -> 555
puts Foo::Bar.new.glorf  # -> 123

Can someone provide some explanation behind why the first call is returning 555 and why the second call is returning 123?