Hello. The reason is because all the lines match from one file to all the other lines in the other file.

If I run your code on two files where some of the lines match and some do not, only the lines that match are printed.

file1
one
two
three
four
five
six
seven
eight

file2
five
six
seven
eight
nine
ten
eleven
twelve

prints
five
six
seven
eight
[download]

update: oh. I didnt think of that SuicideJunkie! if i add a new line between 'three' and 'four' in file one, all the lines are printed out.