That doesn’t increase as you see ads. It increases only as payments are made.
As you said in your topic, your estimated earnings (which is Earnings So far) will grow as ads appear. Then at the end of the month, that will go to 0 because a new month begins.
Brave then reconciles everything regarding ads you’ve seen and what you should earn and prepares payments. Then around the 7th of each month, payments begin to get sent out in batches to everyone. In some months, you may be lucky and get your BAT on or before the 7th. In other months, you be more on the bottom of the list and get it 1-3 weeks after payments begin.
You can always track payment status for the month at Ads Payout Status Update