[EricH]

When running front-back, all 4 wheels are contributing "full power" in one direction. When running side-side, all 4 are contributing "full power" in another direction, but two of them have to work against the other two to do it. This will amplify any effects of friction and other inefficiencies that were masked by running all together.

The problem with theory is that it often leaves out reality. This is why engineers use safety factors and try to account for reality wherever they can. Theoretically, there is no friction and no inefficiency, and inefficiency (where it does exist) is uniform any way you look at it. In reality? Not only is there friction and inefficiency, but it's hard to give them a nice, easy number.