On the page about mistakes in the books, for the chapter 'the Midnight Duel', it says that Charlie might have left the Quidditch team before 7th year, but he was captain. I don't think a Quidditch captain would or could leave the team.
Well I don't know about how true this is, but it would be possible that they didn't win the cup of 1990/91 because he might have had to leave somwhere like a job interview. It's totally theoretical of course but Charlie was training dragons (as a stable full time job) when Harry and Ron got to Hogwarts so maybe they've had some sort of prior testing. Maybe having candidate/s looking into the job on the weekends. Like an internship or something. Due to that he wouldn't have been able to participate in Quidditch (or at least not always) robably giving up his position as captain. Of course the team wasn't thrilled about this and it was when Charlie left the team (I'm not sure what exactly stands there about what he left, just that he left)