I understand that the limitation is not the number of slots, but the amount of flash memory which PSR9000 has for storing the user styles (2 Meg or so). If you use small styles (20 KB average), then you can make use of all the 100 slots. The styles which come with the instrument must be bigger, because the 85 custom styles take up most of the flash memory. Therefore, if you want to add an extra custom style, you must free up some memory by deleting the existing style(s), but this is not a one-for-one deal, as deleting one large style may allow enough room to add two smaller ones.
BTW, was it you at NAMM demoing Solton? or was it another Dan O'Neil?