There has been a general inflation in magazine "publication dates". People don't want to buy a magazine that is "nearly out of date", so towards the end of May, they stop buyin the May edition because the June one is surely going to come out.soon. So the publichers defend themselves against that by publishing the June one in late may, so people will be pleased to have got the June one early instead of bored with the May one late. But after a while, people catch on, so the publishers put the June editon out earlier and earlier, until it reaches the beginning of May - so that it "expires" in early June. They don't go earlier because the June edition wouldn't be valid for any of June at all.