They issued their preliminary PC sales estimates for the first calendar quarter of 2013 Wednesday, and while they agree that it represented either the worst quarterly decline on record (IDC) or the worst in a generation (Gartner) — thanks in large part to the disruption caused by the iPad and its imitators — they can’t seem to get their stories straight on the sales of Apple’s (AAPL) computers.

• According to Gartner, Mac sales in the U.S. grew 7.4%
• According to IDC, they fell 7.5%

We’ve seen the two firms get their Mac signals crossed before, but never this badly. In the summer of 2012, IDC had U.S. Mac sales falling 1.1% in the second quarter while Gartner had them growing 4.3% — a 5.4% gap. This time they disagree by nearly 15%.

You used to be able to explain these discrepancies by the fact that Gartner reported shipments to end users and IDC shipments into channel (i.e. distributors). But now they both claim their numbers include shipments to end users, so that explanation doesn’t wash.

What all this really shows is that unless these celebrated firms are working from sales figures supplied by the companies themselves, they’re giving us numbers that look solid and definitive but are more like data soufflés. Kick them a couple times and they’re liable to collapse.

In the PC market, where the largest players report unit sales on a quarterly basis, things eventually shake out.

But in the increasingly important smartphone and tablet markets, in which Apple issues unit sales numbers but its largest competitors don’t, Gartner and IDC — and all the analysts who depend on them — are flying blind.

Below the fold: the two firms’ Q1 2013 spreadsheets, for what they’re worth.

Gartner:

(Apologies for the bizarre totals in Gartner’s U.S. numbers which, as reader Amir Amir points out, make no sense, even as numbers.)

IDC: