how is work SG level ?

Your contribution value (CV; "level") is coupled to the price of the game on Steam. So if a game get cheaper on Steam (no sale but permanently), your CV will decrease.

Edit: + the bundle list is kept manually, so if there is a new bundle, you give away the games in the bundle at once, but the bundle list is only updated a couple of days later, your CV will decrease then.

Edit 2: Also read "Contributor Value and Levels" in the FAQ.

è un po' un caos!!! xD
comunque sicuramente presto ti risponderanno con un sacco di link e informazioni! prova a dare un'occhiata a questo topic ad esempio: https://www.steamgifts.com/discussion/lMxWN/how-did-i-lose-my-steamgifts-level
o questo: https://www.steamgifts.com/discussion/gq18J/lost-contributor-level
ne so qualcosa perché io sono sceso mille volte da 9 a 8 e a 10 non arriverò mai ahaha =P