ruby strange output

subtract.rb

p1 = File.open('people1.txt').readlines
p2 = File.open('people2.txt').readlines
puts p1 - p2

=begin

ruby-1.9.2-p136 :006 > f1 = File.open('people1.txt').readlines
 => ["tatiana\n", "ana lucia\n", "sergio ney \n", "marco aurelio \n"] 
ruby-1.9.2-p136 :007 > f2 = File.open('people2.txt').readlines
 => ["tatiana\n", "cantido \n", "marco aurelio\n", "renata"] 
ruby-1.9.2-p136 :008 > f3 = f1 - f2
 => ["ana lucia\n", "sergio ney \n", "marco aurelio \n"] 
ruby-1.9.2-p136 :009 > f4 = f2 - f1
 => ["cantido \n", "marco aurelio\n", "renata"] 
ruby-1.9.2-p136 :010 > f1 = File.open('people1.txt').readlines
 => ["tatiana\n", "ana lucia\n", "sergio ney \n", "marco aurelio \n"] 
ruby-1.9.2-p136 :011 > f2 = File.open('people2.txt').readlines
 => ["tatiana\n", "cantido \n", "marco aurelio\n", "renata"] 
ruby-1.9.2-p136 :012 > f3 = f1 - f2
 => ["ana lucia\n", "sergio ney \n", "marco aurelio \n"] 
ruby-1.9.2-p136 :013 > f4 = f2 - f1
 => ["cantido \n", "marco aurelio\n", "renata"] 

=end