Because there will eventually be 6 east/west runways. FAA allows the numbering like that to avoid confusion. The north set are numbered 9/27. L/R exist now, C in the future. The south set numbered 10/28, just one now, second will be done next year (it's the point of the article at the start of the thread), last one will be the last one done.

Same thing exists at other airports too. ATL has 2 8's, 2 9's and a 10 that are 9's. LAX has 2 6's and 2 7's that are 7's. DEN has 2 16's and 2 17's that are 17's *and* a 7 and an 8 that are both 8's.