I'm not sure if this is the case, but sometimes this has to do with 'backwards compatibility'. I've seen databases where some passwords where MD5, some SHA1, some Bcrypt and so on. The login page then will do something like:

  if(checkBcrypt(password)) {
    login();
  } else if(checkSha1(password) || checkMd5(password)) {
    updateDbPassword(password);
    login();
  } else {
    loginError();
  }