The generalization that it's "The same as all of them, differential thrust", is still wrong.
First of all, most quadcopters don't use differential thrust to create yaw. Increasing the rotational speed of two rotors and slowing the others causes yaw due to conservation or rotary motion. In the process of changing rotor speeds on those fixed pitch props, they do change thrust, but that's not what causes the yaw of the vehicle.
Secondly, in the example of the variable pitch, glow quadcopter, again in the process changing pitch of 2 rotors and reducing pitch of 2 rotors does create differential thrust, but it's not the differential thrust that creates the yaw. In the case of a rotor aircraft, thrust is in a downward direction. Differential thrust makes you go up and down. Or increase some, lower others, differential thrust can hold the same altitude.
Let's take an extreme example. This won't create any lift but consider what would happen if the two clockwise rotors were set at 90 degrees pitch (straight up and down) and the two counterclockwise rotors were set at zero pitch. Now power that crazy thing up. What will happen? Seriously, think about it for a minute before reading on.
First consider that all rotors are rotating the same speed. This means that there is no vehicle rotation due to different rotor speeds. The feathered counterclockwise rotors aren't creating much in the way of other forces. The clockwise rotors are beating the heck out of the air, pushing air and creating lots of drag and the create the same force all the way around their path. But the forces applied to the airframe are multiplied by their distance from the center of the airframe. So the outer two tips of the clockwise rotors create a force of let's say X which is multiplied by their distance from the center Y. Note that the direction of the force of both outer rotor tips is in the same rotational direction with respect to the center of the airframe. Now consider the inner ends of those rotors. They also create a force of X and it is in the opposite direction of the outer rotor tips. But it has a much shorter moment arm, for our example let's say it's 1/2Y. So the outer tips each create a force of XY in one rotational direction while the inner tips each create a force of 1/2XY in the other rotational direction.
What will that non-flying quadcopter do? It'll rotate in a counterclockwise direction because of those blades slapping all that air.
Now, tone down the exaggeration. Take a balanced, flying, variable pitch quad. Increase the pitch of the two clockwise rotors, decrease the pitch of the two counterclockwise rotors. Yes, there is differential thrust, but it doesn't cause anything because it's balanced. However, the changes in drag will cause the same thing that happens in the above ridiculous example, just to a lesser extent. But it will still cause the vehicle to rotate.
Science is a wonderful thing.
Dave