closed comments getting spam

Old posts’ comments are closed but still, they get spam.
How is this possible?

For example, I’ve just received a comment on this post.

I searched before posting.
It doesn’t seem to be trackback spam and I’ve disabled them:

You can disable pingbacks and trackbacks in Options | Discussion | Allow link notifications from other Weblogs (pingbacks and trackbacks).

Here’s the comment:

A new comment on the post #101 “7-Zip 4 beta available” is waiting for your approval
http://www.nunomira.com/blog/?p=101

Author : Der Blutharsch [EP] (IP: 75.126.156.50 , aj20.info)
E-mail :
URI : http://holidaymp3.741.com/index.html
Whois : http://ws.arin.net/cgi-bin/whois.pl?queryinput=75.126.156.50
Comment:
Der Blutharsch [EP]

news

To approve this comment, visit:
….

I’m using Akismet, so a lot of spam gets caught.
I’ve installed reCaptcha to see if it would put an end to it. But no.
I still get tons of spam on closed posts.

How is this possible?

I’ve read that I can install more plugins. But I don’t want to.
I’m more interested in understanding what’s going on.