This is likely due to unreconciled earnings.
It basically means that you earned from viewing an ad(s), but the ads aren’t reconciled by the time the payment period starts. So for example, if you earn .05 BAT at the end of the payment period, that BAT may take a couple days to be reconciled (maybe “processed” is an easier to understand term) — which means it doesn’t actually count towards what you earned this month and you’ll instead earn it next month.
So the estimated earnings you’re seeing up until this point have included these BAT that were not yet reconciled — so when your final payout balance for the month is calculated, its a mismatch between what was estimated and what is actually earned. The issue I linked to above resolves this, such that it only shows fully “processed” ads/BAT.
Hope this helps.