Worldwide PC sales figures have taken a tumble — and everyone has an explanation why.

Two separate estimates show the same pattern in sales figures for January through March this year, compared with the same period in 2010. IDC estimates there were 80.6 million shipments worldwide, down 3.2%, while Gartner puts the total at 84.3 million, a 1.1% drop on its own figures.

Things are even more dramatic for US shipments, where IDC puts the figure at 16.1 million (down 10.7%,) while Gartner has the same number (though that’s a 6.1% drop on its 2010 figure.)

To clarify a couple of points: shipments aren’t necessarily the same as sales, though in the long run the two tend to be closely related (if models don’t sell, there’s no need to ship replacement stock); and it’s obvious there’s a fair margin of error in the calculations of both companies, though there’s clearly a definite downwards movement.

Is this really significant rather than just a blip? Well, perhaps more than it might seem. The worldwide figures at least should in normal circumstances always be increasing even if demand remains consistent: developing nations get richer, meaning more people are in a position to buy a PC; and the costs of producing technology such as PCs is usually falling, meaning either that prices drop or companies can offer buyers a better machine for the same price.

So what’s the cause? The most obvious factor appears to be the effects of the tough economy on US buyers. Indeed, Gartner noted that the drop in sales to consumers was pretty horrendous, and things would have been much worse if it wasn’t for business sales holding up.

Another factor may be that sales figures this time last year may have been slightly artificially high thanks to the release of Windows 7 a few months earlier: it’s certainly possible a lot of people planning to buy a new machine had put plans on hold rather than buy something running Vista.

But the real X factor is the iPad. Both sets of figures cover all the traditional forms of PC (desktop, notebook, netbook) but exclude tablets. And at first glance the figures add up. The original iPad has sold an estimated 15 million: take away the initial sales bump and you’re probably talking in the region of three million sales between January and March, which is right around the drop in PC sales from IDC.

My gut response is that even if the numbers tally up, it doesn’t sound right: after all, while I expect most of us know somebody who has an iPad, we don’t know many people who’ve gone as far as to ditch PCs altogether.

But a close look at the figures of both companies shows that by far the biggest loser among manufacturers is Acer: a worldwide drop of 12% or 15% depending on the source, and a US decline of either 24% or 42%.

Given Acer’s product lines, that suggests to me that there’s a strong likelihood that iPad sales are coming at the expense of netbooks and the cheaper notebook PCs. In other words, the chances are this isn’t a case of people ditching the “house computer” for a tablet, but rather that the iPad is taking some of the market for the second machine: the one that’s used more for casual web browsing on the sofa than “real computing.”

If that’s the case, then the whole affair may simply be a matter of terminology: while it may be argued that a netbook is a PC and a device such as the iPad isn’t, it’s entirely possible that for the “lost PC buyers” there isn’t any practical difference. And if that’s the case, the number of people buying a processor-based machine that meets their needs hasn’t actually fallen at all.