It could be any one or a combination of these reasons:
Fees: Don't forget that we need to survive! Patreon takes 5% after payment processing fees from Stripe and PayPal. Depending on how you payout, there will be a fee assessed on your payout- You can see a breakdown of these fees for each of your payouts from your financial dashboard.
Declined pledges: Another cause is if some of your patrons' payments declined. You can access your Patron Manager and select any paid post to see if any patron payments were declined.
Pledges to other creators: If you're a patron to other creators, we'll subtract your pledge from your creator payout. This prevents unnecessary additional fees, and your card or PayPal account will be billed less/not at all.