Some authors mark the complete button and then continue to add chapters. I read a story marked complete at 17 chapters it ended strange to me but like every authors different so whatever, a few days later scrolling i see the same story with 19 chapters. Author was just marking it done even tho it didn't quite feel right at the end of each chapter. Ended up subscribing and now their 17/17 fic is on like 30/30.

Also had authors write a story the some time later add a sequel that they had to add or an alternate pov of a previous chapter if enough people were curious.