Imaging Resource publishes an interview with Foveon's GM Shri Ramaswami explaining resolution enhancement claims of the latest Foveon Quattro sensor. Basically, the reason is that the top high resolution layer is "blue-heavy" rather than blue:
"You can see that the top layer is blue-heavy, but it's not blue. The next layer is green-heavy, but it isn't green. The bottom is red-heavy. None of these is just red, green, or blue -- that really allows you to do something very interesting. So this is fundamentally a pretty smart way to keep your information, but at the same time, reduce the whole load on the system, because these things are not pure colors. It may sound counterintuitive, but it actually allows you to separate out very cleanly all the detail information -- as we call it -- from the top layer, and understand where the color detail comes from. In other words, it allows us to actually get back what was apparently lost." Regarding the possibility to enhance resolution with a pure RGB sensitivity, Shri responds "It would be impossible then, without that correlation, to do this."