Hey guys, why is my game is put on Sandpit category, although I selected it as a Mystery ?

Because the moderators decided to put it there. Games put into Sandpit are considered incomplete, tests, etc.

I think all new games go into the sandbox, which will be looked at by moderators, and then moved accordingly, if the game is adequate enough

I think one of the reasons for this is so that the playable games are swamped/deluded with "spam/noobie/incomplete/bugged/etc" games

(if you want people to play your game while it's still in the sandbox, you'll have to provide the url link for it, for people to be able to download your game)

The uncategorized section and the sandnox section are completely different sections.