A lot of people think that 6/8 means 6 beats to the bar and 12/8 means 12 beats to the bar. This is not correct, we are talking about Compound Time here, not Simple Time like 3/4 or 4/4. 6/8 has two beats to the bar - each one a dotted crotchet. Similarly, 9/8 has three dotted crotchet beats to the bar and 12/8 has four. Within those bars, you would group quavers in blocks of three - equalling a dotted crotchet.
What Yamaha and others have done, as has just been said, is to write the 6/8 patterns in 4/4 with triplet quavers. It's actually a 12/8 pattern so counts 1,2,3,4. It makes no difference to the sound at all, and has the advantage that anyone looking at a tempo indication in the music of a 6/8 piece and seeing something like dotted crotchet = 72, can set the tempo on the keyboard to 72 and know that it will be correct. If you have a true 6/8 pattern, then you have to do some mental maths to set the tempo correctly. On my Roland, most of the 6/8 patterns do the same as Yamaha but a handful do not and it is a pain to create registrations that switch from one type of pattern to another as switching between them is unreliable.
So my recommendation is to follow the herd and create all your Compound Time patterns in Simple Time with triplets.
6/8 - use 4/4 and create a 12/8 pattern
9/8 - use 3/4
12/8 - use 4/4
15/8 (rare but it does happen) - 5/4