First of all I want to commend the test engineers who managed to get wind tunnel results on the undried out, non-carcass of the living dragon that was used in the comparison. Well Done!
To understand the aerodynamic aspects of dragon scales and the role that their "slime" plays, you need to look no further than your local craft supply store. The self-healing cutting mats which are used in various blade-wielding crafting techniques are very similar in form and function. In fact, the formula used to create these mats was reverse engineered from actual dragon scales. Like real dragon scales, these mats flatten out and become perfectly smooth no matter how often you cut grooves into them with your crafting knives.
Dragon scales are made of a non-newtonian fluid which remains flat, slick and low-drag when passing through winds while in flight. This eliminates much of the turbulence which would otherwise disrupt and steal energy from the massive creature's flight. The dragon's massive chest basically takes on a teardrop shape while in flight, cutting through the air with minimum resistance. The harder the headwind blows against that amorphous form, the more streamlined the form becomes.
The magic of non-newtonian fluids is that these scales can also serve as armor. They harden to impenetrability when struck at speed. Swift blunt force is forcefully rebuffed by a rock hard surface which was fluid just a moment earlier. A slow blade can wound even a mighty dragon, but when you
consider the dragon's lightning reflexes, you are unlikely to get a chance to move slowly.
So the reason that the scales help in flight is that they conform to resistance of the air to minimize drag. And when, under UV, they dry out to a non-fluid state, that conformation ends, greatly increasing the effective drag upon the dragon's corpse.