Iptables is not blocking an ip?

[chrisl at xp.etowns.net]

At one of my customers sites I have iptables configured to drop all ssh packets unless they originate from one of two addresses. However in logwatch, I had login attempts though SSH. 

 Illegal users from:
    220.193.2.37: 6 times

Now the address above is not one on the allowed list. Is it possible that they were able to get past iptables to attempt the login?