As far as I know it's because SG checks your owned games via Steam API and it doesn't offer a way to check owned DLC. Therefore this issue exists for all DLC and also some Gold and GOTY editions. So when giving away a DLC I would recommend checking the winner's profile first but sadly I know of no way to check someone else's owned DLC. I don't even see if my Steam friends own a specific one. Another thing Steam should (finally) implement.

All this aside: You are here for a month already so "Welcome to SG." would be a little late but what I can say is that it's nice to see someone sharing so much with the community despite being rather new. I hope the winners will enjoy the game and that you will have your first win soon too. :)