In competing reports issued Wednesday afternoon, the two leading PC market research firms reported domestic Mac shipments for the second quarter of 2009 that differed by more than 200,000 units.

Gartner reported Apple (AAPL) shipping 1.422 million Macs in the U.S., a 2.5% increase over Q2 2008 in a quarter that saw overall PC shipments in the U.S. decline 1.2%.

IDC had Apple shipping 1.213 million Macs in the same quarter, a 12.4% year-to-year decline that was far worse than the industry average -3% and dropped Apple into fifth place the U.S market, after Dell (DELL), HP (HPQ), Acer and Toshiba.

According to Gartner, Apple has an 8.7% share of the U.S. market. According to IDC, that share is 7.6%.

The only stated difference between the two reports is that Gartner counts X86 servers as PCs and IDC does not. Apple did introduce a new version of its Xserve rackmounted server in April, but the server market is particularly soft this year and Apple is unlikely to have sold 20,000 units in the quarter, never mind 200,000.

Both reports are labeled “preliminary” — a designation that is supposed to encourage us to pay for their propriety final reports but which does not, in this case, inspire confidence.

“We may revisit our numbers next week, and get more aggressive,” Loren Loverde, director of IDC’s PC sales tracker, told Computerworld when asked about the discrepancy.

Gartner and IDC’s press releases are particularly unhelpful for Apple analysts because they include only the top five vendors in each category. Apple usually makes the top 5 in U.S. shipments but has never done so, for as long as we can remember, in worldwide shipments. For definitive sales figures, we’ll have to wait for Apple’s fiscal third quarter earnings, which are scheduled for release next Tuesday.

IDC and Gartner both report overall PC sales as better than expected, although Gartner attributes that in large part to inventory restocking. IDC believes PC shipments fell 3.1% worldwide in the second quarter, better than the 6.3% decline its analysts expected. Gartner has global PC units falling 5%, but that’s a lot better than its predicted 9.8% decline.

IDC’s Loverde called its results “a very positive indicator for the second half of the year.”

Gartner’s Mikako Kitagawa saw “a small sign of a PC market recovery” in a market that is still in decline.

UPDATE: Found this in The Four Hundred, where Timothy Prickett Morgan was writing about last month’s server sales:

“Gartner measures server revenues and shipments from vendors or their channel partners into end user accounts, while IDC measures vendor factory revenues.”

I’d never heard that. According to reader Joe Wilcox, it could explain the discrepancy. I.e., Gartner’s numbers are higher because of strong Mac sales; IDC’s numbers are lower because of inventory already in channel.

Below: IDC and Gartner’s U.S. and worldwide PC numbers.

IDC:

Gartner: