Steve, I was just afraid of this question!!!
These is one of those things I, or anybody else, can't satisfactory explain.
But, the orchid world is full of these inconsistencies. You see, just to mention an example that comes to mind now, why Cattleya mossiae var. reineckiana is reineckiana and not semi-alba?
Sorry, I'm afraid I can't help this time!!
I guess the orchid world is full of mysteries!! Thanks anyway!!