Admob Native ad is "stealing" an item

So after I've implemented the NativeAds, only 3 days after I've noticed that my random method of displaying the ad is stealing one item from the json.

So, currenlty I'm displaying the ads every 13 items

if (index % 13 == 0)
            return adsContainer();

So this is stealing my 0 of json.

What could I do to stop this?