Really, I do not know how I missed that :) I read it a couple of times to and I still thought he wanted it the other way.
So my original comment kind of stands but in a opposite way.
I have never see file_9.txt sorted before file_10.txt, I just tested it on OpenBSD and I got this, which I have always seen:
$ ls|sort
file_1.txt
file_10.txt
file_12.txt
file_2.txt
file_20.txt
file_3.txt
file_9.txt