The top item on the closely-watched Techmeme news aggregator Saturday morning was a piece by ZDNet‘s Larry Dignan about how the shipments of tablet computers have failed to meet the industry’s “lofty expectations.”

It was a theme picked up by at least eight other news outlets, including VentureBeat (“Tablet sales slow”), The Loop (“‘media tablet’ market isn’t as strong as previously thought”) and Forbes (“Sales Dip Hints Media Tablets Won’t Replace PCs Any Time Soon”).

The source for all these pessimist headlines? An IDC report that the total number of tablet computers shipped into sales channels in Q1 2011 was 7.2 million.

At first that struck me as absurd, given that Apple (AAPL) alone is expected to report next week that it sold a good deal more than 7.2 million iPads last quarter.

Then I took a second look at IDC’s report. What I missed the first time — and what these reporters failed to take into account — is that IDC was talking about Q1 2011, which runs from January to March, not Q2 2011, which ran from April to June.

Of course tablet sales dipped after the October-to-December holiday quarter. We knew that months ago. This is news?