The original skateboard mounting hole pattern had the bolts set farther apart than the current one. When nose and tail slides started becoming a big thing in the early 90s street skating, it made more sense to set the bolts closest to the nose tail a bit further back so they wouldn't interfere with the slide. I remember reading somewhere that the Wray bros. were among the first do this. However, not all decks were completely switched over to the "new" bolt pattern so a six hole baseplate would let you use either the old pattern or the new pattern to mount your trucks. Eventually, some truck brands did away the six hole baseplate in the mid to late 90s.
During the early 90s there were also double drilled decks to adjust the wheelbase (i.e. longer or shorter). These are sort of making a comeback with some small, underground type deck companies. Also, there is one brand of truck, Polar Bear, that has an eight hole baseplate for wheelbase adjustment and the old school mounting pattern.