flyback multi output
This effect is called cross-regulation.
Normally only one output is regulated (the master) and the rest are just supposed to track. In a flyback the most you can hope for is about +/- 6% on the slaves, but more likely +/- 8%.
If better regulation than that is required, then you need to add post-regulators.
The causes for cross-regulation are: transformer leakage (less than ideal coupling), voltage drops in transformer windings and diode voltage drops which change with the load currents. All of these lead to poor cross-regulation.
Good transformer winding requires tight coupling between secondaries, but this is not easily achieved, especially if the number of secondaries is large.
Ensuring secondaries have low DC resistance is generally accomplished by using stranded wires, since you cannot just increase the diameter, given the high frequency.
To minimize rectifier drops you can use synchronous rectifiers, but that will add to the cost.