On tarmac, a tire that isn't slipping isn't generating force. That's the assumption underneath essentially every road-racing tire model, and it's a good one on tarmac.
On dirt it's false. A wheel rolling across loose ground is displacing material and packing it down the entire time, which means it's being resisted before any slip happens at all. And it's not symmetric; dirt under a driving wheel behaves differently from dirt under a braking one, so the car has genuinely different characteristics on the throttle and on the brakes.
Raw R/C's off-road tire model has both of those properties built in (and a lot more!), because it was designed for dirt rather than adapted from tarmac. It's why the car bogs in the loose stuff, why it doesn't coast the way a road car does, and why the throttle is a handling control on dirt rather than just a speed control.
