Because the characters went through 5 years in between Soul Key and the TP pact novels.
Four years, actually. I think this misconception exists because Rough Beasts of Empire
spans about a year and the rest of the books in the first TP miniseries take place toward the end of that year. So yeah, there are about five years between The Soul Key
and, say, Zero Sum Game
or Paths of Disharmony
, but only four between TSK and the start of Rough Beasts of Empire
, which is where we pick up the story of Sisko and the other DS9 characters.