How come some question there is a reply option and some don't

When the OP select a answer , the post is close.

If op choosed best answer already, you can only view but can't reply

ditto... as a test, select best answer and you will see the magic happen :P

No reply button if best answer was choosed, click green button below and you will see.

If someone ask a question and choose an answer as the best one, it'll close the discussion. The Whizz can choose an answer as well and close the thread. Then the "Reply" button will disappear.

If someone selected an answer as a "best answer", it's not possible to reply to this quesiton anymore. It's closed.

Once a "best answer" as been selected than the reply option disappears or the OP has closed
you will no longer be able to reply to this post