:::Ah, interesting. I knew about the American Code (and indeed later disputes between US and European taxonomists, e.g. over Hippeastrum vs. Amaryllis). Not sure where, if anywhere, Canadian taxonomists stood! 
 :::Whoever wrote the bit on the IPNI page here seems to be of the view that the Kew Rule was knowingly deviant by writing "Jackson (1887) described and   	  	 his use of this rule" [my italics]. I can't find what "Jackson (1887)" refers to. Jackson may have felt it necessary to defend the Kew Rule given that de Candolle had objected in 1877; perhaps de Candolle in 1888 was responding to this piece by Jackson. 
 :::There is a logic to Jackson's argument. Suppose the names A b X and C d Y are independently published for the same species (homo- or heterotypically), and it seems that C (perhaps narrower in concept) is the better genus, then someone else has to correct C d to C b (X) Z, so there are now 3 synonyms to keep track of, instead of 2 under the Kew rule. On the other hand, keeping the epithet constant is very helpful in researching synonyms.