IIRC, the logic goes something like this:
You put out the magazine early in March, with a March cover date. Few weeks later - and by now we're into April, just before the next one is due -, Joe Public alights from the Clapham omnibus, and goes into his local newsagent to peruse the latest periodicals.
"Aha!' quoth he, "There's my favourite magazine!" After leafing through Railway Modeller for a while, he puts it back on the shelf with a sigh, and spots What Hi-Fi? Sound and Vision.
But it says March on the cover, so he leaves it, thinking it's an old one.
However...
Put it out in March with an April date on it, and not only do newsagent browsers pick it up in March, as it's "next month's", but you also get sales at the end of the sales period, as it still has the current month on the cover.
As the meerkat would have it, 'simples'.