Difference between plain print, and print with format string?

Wed Jan 16 14:24:10 EST 2002

I noticed that in my python cgi script, the following comes up in
Netscape 4.7 as plain text:

--------------------
def htmlHeader():
  return "Content-type: text/html\n"

...

print htmlHeader()

print <html tags and stuff here...>
----------------------

This works in IE 5.5, but just dumps the plain html in Netscape 4.7. 
Choosing View | Page Info shows the "File MIME Type", according to
Netscape, as "text/plain".  Changing the line above to the following
fixes the Netscape issue:

print "%s" % htmlHeader()

So, what's going on here?  What's the difference?

Thanks,
Chris