Assume your promotion includes the following:

If you increase quantity for the red blouse by 100, you now have the following:

If you now increase the quantity for the generic blouse by 100, you have the following:

If you now increase the quantity for the blue blouse by 100, you have the following:

In this last case, the increase is not distributed. Redistribution only occurs when you change the generic article quantity directly, not when the change results from a change in a variant quantity.