This is because it will redistribute the subtracted weight among the other bones, usually in ways you don't want. It's better to paint positive weights to the intended bone and have it steal the weight it needs.
1. Workflow-wise, it's better to assign weights to their intended bone than to subtract. For example, if the Neck is influencing the Head too much. Don't select the Neck and remove weighting. Select the Head and add weighting. This will keep things much more predictable. 2. Is this for a game engine? Couple of things.…