As Joe H says, maybe the added X is intended to indicate an eXpanded set of features, or that the board is loaded with eXtras, or that it's just seXy, but the main benefit of the naming convention is that Yamaha can now re-use the 900, 910, 950, 970, 975 and 700, 710, 750, 770 and 775 series. That solves their mid-range naming challenge for the next dozen years or more with little effort while maintaining a historical connection to a group of now "Classic" models.
j.