Gmail amazing shrinking spam folder

I've had a gmail account for a while now and after getting acquainted with it I have moved all my personal mail to it. I have a vanity domain that I've used for several years that I configured to forward to the gmail account. I did this after gmail started offering name spoofing so that my return address would remain as my vanity domain.

Of course having a long used email address meant I immediately had a significant spam load. Gmail's spam filters worked pretty well and the 100 or so spams a day were all going to the spam folder. Like anyone with sense, as I was learning to trust Gmail, I would check on the contents of the spam folder every day or two. But I recently stopped bothering. Gmail automatically deletes spam that arrived more than 30 days ago and the spam folder contents don't count towards the space limit.

So here's the odd thing. It's been about two, maybe three months since I stopped examining and deleting the spam. For the previous several months, perhaps year, that I used gmail I was at a pretty steady diet of 100 spams a day. Plus or minus maybe ten percent. And for the first month I saw the spam box build up to around the 3,000 count that I expected. But for the last month, the total spam count sitting in the spam folder has been steadily shrinking.

Looking into the spam folder I see that a month ago there are about ... 100 spams per day. But over the last few days the number is under twenty per day.

Not that I'm complaining or anything but ... where has all the spam gone? I can think of several possible explanations:

1. The ecomonic model that supported spam has change sufficiently that the amount of spam on the network really is going down. Doesn't jibe with other articles I see that list total spam as the lion's share of all email on the 'net.
2. Gmail has instituted some global "front door" filters that stops more spam before it gets to my inbox.
3. Gmail learns my spam by my leaving it in the spam folder instead of deleting it and then filters more before storing it.
4. Lack of response from my email address is causing that address to fall off the spammers' lists.

But of course, writing about things causes me to think about them more. And there is another possibility. I used to have a "catch all" forwarder on my vanity domain. So any random email address at the domain would get forwarded to my real address. Some time back I gave that up as a bad idea and now only forward selected addresses. So I guess that's the real story. Eighty percent of the spam I was getting was being sent to addresses that were guessed or simply made up.