That`s because there isn`t a universal definition of flop. If you gained exposure with an Oscar nom or win (Kate Hudson,Brody, Jen Hudson) and never come close to it ever again and your movies are considered garbage in averige or you are on Smash which is,like, bottom of the barrell and than some than you are a bone fide flop in your category.
Same with people hyped to be next leading man aka boxoffice such as Borelando. Classic flop.