I think they specifically made it not mark off already called numbers... just as a penalty for joining the game late. Previously you could not join late at all.
takkewyf, yup, DL is trying to make money somehow on this otherwise "free" game... but I don't fault them for that.
It's not really been made that way as a penalty for joining late, it's more to stop cheats. User joins game at start, as bad cards with hardly any numbers marked, quits, rejoins and gets better cards that are then auto marked and goes on to win.
That's the case that DL was stopping from being able to happen by only making it dab cards from after you join.