A lot of times, in a carbureted engine, the oil will smell like gas because it probably does get a little gas in it, over time, especially if it's not jetted right or if it floods sometimes. A carbureted engine is not nearly as efficient as a fuel injected engine. Fuel injection is going to be more accurate at delivering the correct amount of fuel to the engine than the carb is. In a carbureted engine, any "extra" fuel the engine receives is going to end up going by the rings down into the crankcase into the oil. That's why it's important to keep the oil changed often, especially if it's "flooding". Fuel injection is more accurate and there won't be as much "extra" fuel to go by the rings.
That being said, I doubt that fuel injected ATVs are hardly as efficient as automobiles, because automobiles have a closed loop system and I believe the ATV fuel injection is an open loop system. In a closed loop system the PCM controls the amount of fuel delivery through the fuel injectors and can vary it depending on the sensor readings, such as the oxygen sensor. By varying the fuel delivery according to the oxygen sensor, the fuel mixture can be kept closer to the "ideal" mixture.
You may remember before automobiles went to fuel injection, when a vehicle got around 100,000 miles on it, it was pretty much done. Since fuel injection came into the scene, now you can get at least 200,000 or more on just about any vehicle that has been taken care of. A fuel injected engine doesn't get all the "extra" fuel in the cylinder to go by the rings and wash the oil off the cylinder walls, causing the walls and rings to not get properly lubricated.