Hey @good1 ! Welcome to the community 
The way Brave Rewards work is that every month, your Estimated Earnings reset to 0, representing a new month of fresh earnings.
The issue is that in recent months the Estimated Earnings counter has not been resetting properly and your ad earnings in one month would carry over into the next month making the counter either not reset or decrease unexpectedly.
You can see more about this in the post here → Heads up: Estimated pending rewards counter fix
Hope this helped!
Porridge