It's a publishing tradition, the psychology being that it you go into the shop in the middle of the month and see a magazine with that month's date on it, you'll assume it's old, and there'll be a new one soon. If it has next month's date on it, you're more likely to buy it.
That said, things get a bit skewed around here at this time of year because we publish 13 issues, not 12, a year the usual monthlies plus an Awards special (on sale in four weeks' time, by the way). So publishing every four weeks rather than once a calendar month means the issues get earlier running through the summer and then kind of go back to normal - Awards is on sale 15th Oct, December 11th November.