How are people accessing my "thank you" page without my aweber subscriber count increasing?

Here's something strange that's happening with a new list I am building. I hope someone can help me out:

I am advertising my squeeze page on 6 or 7 different safe lists. Clicks are coming in steadily. The URL of the squeeze page is /go. Once I collect a name and an email address, my subscriber count goes up by 1, and they are redirected to /lesson1 to begin the program. I accomplished this redirect easily using aweber's "thank you page" URL setting.

FYI: I am *not* using double opt-in.

That's how it is supposed to work... and that's how it works every time aweber or I test it out. But here's what I am noticing in my wassup logs (access logs):

People are in fact hitting /go from the safe list URL, and then a few seconds to a minute later they are accessing /lesson1. Seems fine, until I check aweber 10 minutes later and NO new subscribers are captured. I thought it was a glitch, so I waited for more evidence: there have been 20 or 25 unique IPs that are having the same thing happen, and aweber confirmed these IPs are NOT already on my list.

So how the heck are people getting redirected to the thank you page without aweber capturing their info and adding +1 to my list?