The style time signature is of course compound 2/4 (bear in mind the restrictions of the style engine in the first place) - the definition of what that time signature means is debatable concerning modern pop music and style management - indeed one can look at classical compositions and ask the same question - e.g Why does Bach (J.S) define 3/8 against 3/4 or 6/4 against 6/8 (not to mention 4/16 or 9/8 when the sub-rhythm is predominantly perceptually different to our more contemporary hearing).

I would expect the inclusion of 6/8-defined styles to be relevant to market research, carried out amongst various subsidiaries, within Yamaha Corporation's naming decisions, no doubt sourced by equivalent references.

Just my own supposition

All the Best

Jacob