Here’s what I don’t understand about the Forrester Research report that got so much press on Thursday — besides the fact that it takes Microsoft’s (MSFT) Steve Ballmer’s side over Steve Jobs’ on where the PC market is headed. (Jobs compared PCs to farm trucks; Ballmer called the iPad a PC, and Forrester’s Sarah Rotman Epps agrees.)

The item that got the most attention, however, was Epps’ prediction — bullet-pointed in her summary — that tablet computers would cannibalize netbooks and overtake them in sales by 2012.

But in the text of the full report, and in the accompanying chart, Epps estimates that U.S. sales for tablet computers from all manufacturers will total 3.5 million in 2010.

In other words, in Forrester’s opinion, the 2 million iPads Apple sold in April and May were a fluke. It will be lucky sell 1.5 million in the U.S. between June and December — or fewer than 215,000 per month.

“Consumers didn’t ask for tablets,” she points out in her summary. “In fact, Forrester’s data shows that the top features consumers say they want in a PC are a complete mismatch with the features of the iPad.”

When you look at it that way, it’s a miracle Apple (AAPL) sold any iPads at all.

[Follow Philip Elmer-DeWitt on Twitter @philiped]