It's mostly imagination. But, also
some differences in bike geometry.
In mountain bikes you generally have lots of leverage over a long lever so steering inputs are easy & smooth over a great distance. This is done for a minimal effort & maximum forgivness of controls at the generally slow speeds a mountain bike usually travels at.
A road bike is designed around much higher speeds. So, narrower bars for less leverage & the concept of counter-steering is employed for smooth, stable, planned high speed curves.
If a road bike had mountain bike geometry it would be very mentally fatiguing to ride at very high speeds for very long. It would be neigh impossible to track a straight line with all the thousands of tiny micro over-corrections the uber light steering would cause.
My guess is you are experiencing a differential between expected influence of input to the bike & expected results. So it "feels" unstable because by the time you realize your micro-correction isn't enough, it now needs to be a major correction. Try "relaxing" into it & steering not with your hands so much as with your hips...Look where you want to be. You'll get it.
