Think about it from the browser's point of view. For readonly the browser will send in a variable/value pair. For disabled, it won't.
Run this, then look at the URL after you hit submit:
<html>
<form action=foo.html method=get>
<input name=dis type=text disabled value="dis">
<input name=read type=text readonly value="read">
<input name=normal type=text value="normal">
<input type=submit>
</form>
</html>