You might see different "last" issues of the newsletter depending on your internet service provider and the place in the list.
About a year ago, I noticed that a lot of e-mail is not being delivered. Because of all the spam out there, legitimate newsletters also get trapped in spam filters or simply get discarded by the internet service providers.
This gets especially bad if you send a lot of messages in a short period of time containing identical link to the same web page.
If a large e-mail provider (like Yahoo or AOL) receives 100 (or so) identical e-mail messages in a day, they would stop accepting any more e-mail from that source. So any subscriber after that gets cut off and doesn't receive an issue at all.
As a result, I started spreading the delivery of each new issue over about one week interval, so there are no peaks and e-mail doesn't get blocked.
On top of that, I started shuffling the issues so that they don't all contain the same exact link to one page. So this week, someone might receive an issue that you'll get next week, and at the same time, they'll get your issue next week. And if you signed up recently, you might even get an issue from two months back if there are a lot of other subscribers in the list using your service provider. As long as an issue is not marked as time-sensitive, it might get shuffled around a bit.
It's pretty complicated and took me a while to figure out how to do that without getting confused about who received what, but it turned out to be working pretty good.
There are several attempts by large internet providers to standardise e-mail authentication so that legitimate newsletters can identify themselves and end up being delivered without problems. But until that happens, we have to keep doing some shuffling. Otherwise, most people wouldn't get the newsletter at all.
It's really a pain in the a$$ to maintain a list this way. But there is currently no way around it.
I hope that clears it up.