Numbers might also be skipped because of deleted questions. The ID is probably auto-incrementing so will just keep going up without filling in the gaps. As your previous question showed, it doesn't matter since there is scope for billions of questions/answers/comments.
Re using the same table: I assume it's because questions and answers (and comments to an extent) share much of the same data structure, e.g. ID, text, votes and so on. So it's simpler to put them in one single table.