As I recall, they were starting to get some flak from readers about all of their sensible comparisons (i.e., "Ten Great Sedans under $30,000" and the like). So they came up with one completely insane one.
Also, with all of the utterly insane work the RS8 had done--like the Harmon Kardon GPS system, 5.0 Cammer crate engine, and custom rear suspension--tacked onto the Focus RS's base price, it and the Lamborghini were in the same price range.
I think that after the RS8/Gallardo comparison, Motor Trend followed with a few more based entirely on the price and not what sort of car they were. The only one I can remember is some Mercedes-Benz sport sedan vs. two Pontiac GTOs.