With magazines (and books), there are “on-sale dates.”
Some of them are “strict on-sale” which means that absolutely, under no circumstances, ever, should the book-store sell this book/magazine before the strict on-sale date. Think Harry Potter. This is reserved for big name, big hype authors, and magazines like Newsweek, People, etc. Weeklies that always come out on the same day.
Then there are the other books/magazines that just have regular on-sale dates. Because there’s less hype, the bookstores shelve them within 24 hours of receiving them, instead of waiting for a specific date to do so.
Stuff ships out to the stores before the on-sale dates. The “official dates” are probably to cover the magazine company’s butts. That way if it doesn’t hit newstands at the same time all over, they’re okay–as long as everyone has it by the official date.