The ad auction is like a traditional auction, the more advertisers bid to appear on your inventory, the higher the competition for your ad unit, and so does revenue.
Because it's an auction, AdSense determines bid winners based on the highest actual bid submitted. This true bid takes into account any adjustments AdSense may make to advertiser bids to optimize the auction. If your publisher is still having a headache about revenue decline, you can try this feature on AdSense
Gain visibility and grow your ad revenue with programmatic pioneers, Publift