You used to be able to explain the discrepancies between Gartner‘s and IDC‘s quarterly market share reports by the difference in their methodologies.

Gartner counted sales to end users. IDC counted sales into “channel” — i.e. devices sold to stores and other distribution points, but not necessarily to customers.

But when trying to understand the discrepancy between Apple’s (AAPL) U.S. market share in the reports the two firms filed Wednesday — 10.6% (up 3.8%)  according to Gartner, a round 10% (up 5.1%) according to IDC — we noticed this footnote to IDC’s spreadsheet:

Shipments include shipments to distribution channels or end users. (emphasis ours)

That doesn’t help. If IDC is counting both channel and end users (or is either?), how does it come with a Mac market share that is smaller than Gartner’s?

The discrepancy is even worse for the Acer Group. Its U.S. market share grew -3.8% according to IDC and -25.9% according to Gartner.

For the record, HP was the big winner domestically according to both firms’ spreadsheets, with a share up 6.6% to capture 29% (Gartner) or 28% (IDC) of the U.S. PC market.

Worldwide, the PC market actually grew a couple percent last quarter. It had been expected to shrink 1.2% according to Gartner and 0.9% according to IDC.

Below: The firms’ spreadsheets for worldwide PC sales for Q1 2012.

The press releases are available here and here.