Why does output for `Open3.capture2e("ruby", <file>)` and `Open3.capture2e(<file>)` differ?

parrot.rb

#!/usr/bin/ruby
=begin
This is a simple executable that just parrots what you type back at you, on stdout:
  $ ./parrot.rb
  > foo
  ! foo
  > bar
  ! bar

PUZZLE: Why does output for Open3.capture2e("ruby", "./parrot.rb") and Open3.capture2e("./parrot.rb") differ?
  e.g. ./parrot.rb       ### only prints the script's stdout
       ruby -ropen3 -e 'puts Open3.capture2e("./parrot.rb", stdin_data: "foo\nbar\n").first.inspect'
       =>  "! foo\n! bar\n"

  e.g. ruby ./parrot.rb  ### prints both stdin and stdout 
       ruby -ropen3 -e 'puts Open3.capture2e("ruby", "./parrot.rb", stdin_data: "foo\nbar\n").first.inspect'
       => "> foo\n> bar\n> ! foo\n! bar\n"
=end

require 'readline'

loop do
  cmd = Readline.readline("> ", true)
  exit if cmd.nil?
  puts "! #{cmd}"
end