It's called a quarterly.

No, it's definitely monthly.

It's four weekly.

https://www.practicalfishkeeping.co.uk/store/back-issues/practical-fishkeeping

It's mainly for the benefit of WH Smith. Nobody likes buying old magazines, so releasing the "November" issue in September extends the shelf life. Also, international subscribers feel like they're getting the latest issue even if it takes six weeks to arrive by surface mail.

The cover date is for the newsagent, not you. It's the date when they're supposed to withdraw the issue from sale. Typically a monthly magazine will publish either 12 or 13 issues per year (the latter usually includes a Christmas issue). In this case, the November issue goes on sale from 25/09, and will be removed from the shelves around 01/11, by which time the December issue should have turned up.