Board Thread:General MK Discussion/@comment-30711175-20180507212443/@comment-30104039-20180508120829

Riley Heligo wrote: From my own knowledge, it stems from how MK 2011, was referred to as MK9 by Ed Boon a few times when asked about what fans should call it, and the game prior to that was MK vs DCU, while not a canon game it's still the 8th main game in some context.

And obviously how MKX is referred to the tenth, with X being the Roman numeral for 10. Then he would probably ask why MK 2011 is considered MK9.