why did my level decrease?

bad karma

Some of the games you have given away lost some or all of it's value.

Sometimes it takes several days for games in a current bundle to be added to the bundle list. A game on the bundle list only gives 15% of contribution value, so it is possible that the addition of games in current bundle to the list resulted in your level dropping. Note that when a game is added to the bundle list, all giveaways for the game from after the bundle was released will be affected.

For example, if a game is in a bundle on Jan 15, and is only put on the bundle list on Jan 30, after Jan 30, every giveaway made for that game starting Jan 15 will retroactively only give 15% value.

Another possible cause is a price drop of one or more games you gave away.
https://www.steamgifts.com/discussion/eod99/a-note-regarding-cv-and-price-drops

If you have created a giveaway for a "No Value Game", keep in mind that it might be because of the delay it takes us sometimes to add it to the "No Value Game" list. If a game is given out for free in a promotion, when we register it into the site it get's retroactively added to the "No Value Game" list just as a bundle game would.

For example, if a game was given out for free in a promotion on Jan 15, and is only put on the "No Value Game" list on Jan 30, after Jan 30, every giveaway made for that game starting Jan 15 will retroactively only give 0% value.

You can read all about this new update for giveaways for previously free games here: https://www.steamgifts.com/discussion/RCZPq/reduced-cv-and-no-cv-giveaways

You can find more informations about Contributor Level here and here.

And don't want to sound mean, but there is a search function on these forums :P
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