It's usually filled with articles, how-tos and reviews from previous issues.
You're thinking of our special issues, which were quarterly but are now six times a year. Those are themed issues with, as you say, articles, how-tos, and reviews from previous issues. They marketed toward nonsubscribers, and they're not included in the subscription.
The regular magazine, the one you get with your subscription, is published 13 times per year
. January-December, followed by Holiday, followed by January again.
Why they sell 12- and 24-month subscriptions of a magazine that comes out 13 times a year, I have no idea. I just work here.